How to Scan Computer for Viruses

Computers are essential tools in modern life—used for work, communication, education, entertainment, and financial transactions. But with increasing connectivity comes greater exposure to digital threats. Viruses, malware, ransomware, spyware, and other malicious software can infiltrate your system without your knowledge, compromising your data, slowing performance, stealing sensitive information, or even rendering your device unusable. Knowing how to scan computer for viruses is not just a technical skill—it’s a fundamental practice for digital safety.

Many users assume that having antivirus software installed is enough. While it’s a critical first step, regular scanning, proper configuration, and proactive habits are what truly protect your system. This guide provides a comprehensive, step-by-step approach to scanning your computer for viruses, covering everything from initial detection to long-term prevention. Whether you’re using Windows, macOS, or Linux, this tutorial will empower you with the knowledge to safeguard your device effectively.

Step-by-Step Guide

1. Identify Signs of Infection

Before initiating a scan, recognize the warning signs that your computer may be infected. These indicators can help you determine whether a scan is necessary and how urgent it is:

• Unexplained slowdowns: Your computer takes significantly longer to boot up, launch programs, or open files.
• Pop-up ads: Frequent ads appear even when you’re not browsing the web or using a browser.
• Browser redirection: Your default search engine or homepage changes without your permission.
• Unknown programs: You notice unfamiliar applications or icons on your desktop or in your Start menu.
• Disabled security tools: Your antivirus or firewall has been turned off without your action.
• High CPU or disk usage: Task Manager (Windows) or Activity Monitor (macOS) shows unusual resource consumption by unknown processes.
• Files disappearing or being encrypted: Documents, photos, or other files vanish or are renamed with strange extensions.

If you observe any of these symptoms, it’s time to perform a thorough virus scan. Do not ignore them—delaying action can lead to irreversible damage.

2. Disconnect from the Internet (If Suspicious Activity Is Detected)

Before scanning, consider disconnecting your computer from the internet—especially if you suspect active malware communication. Many malicious programs rely on internet connectivity to send stolen data, download additional payloads, or receive commands from remote servers.

To disconnect:

• On Windows: Click the network icon in the system tray and select “Turn off Wi-Fi” or “Disable Ethernet.”
• On macOS: Click the Wi-Fi icon in the menu bar and select “Turn Wi-Fi Off.”
• Unplug the Ethernet cable if you’re using a wired connection.

This step prevents the malware from spreading or communicating during the scan. Reconnect only after the scan is complete and threats have been removed.

3. Update Your Operating System and Security Software

Outdated software is one of the most common entry points for malware. Cybercriminals exploit known vulnerabilities that have already been patched by developers. Before scanning, ensure your system and security tools are up to date.

On Windows:

• Press Windows + I to open Settings.
• Select “Update & Security” > “Windows Update.”
• Click “Check for updates” and install all available updates.

On macOS:

• Click the Apple menu > “System Settings.”
• Select “General” > “Software Update.”
• Install any pending updates.

Next, update your antivirus or anti-malware software. Open the application and look for an option like “Update Definitions,” “Check for Updates,” or “Update Virus Database.” Most modern tools update automatically, but manual verification ensures you’re protected against the latest threats.

4. Boot into Safe Mode (Recommended for Severe Infections)

Safe Mode is a diagnostic startup mode that loads only essential system services and drivers. Many viruses and malware programs run in the background as startup processes or services. Booting into Safe Mode prevents these from loading, making it easier to detect and remove them.

How to Boot into Safe Mode:

Windows 10/11:

• Press Windows + I > “Update & Security” > “Recovery.”
• Under “Advanced startup,” click “Restart now.”
• After rebooting, go to “Troubleshoot” > “Advanced options” > “Startup Settings” > “Restart.”
• Press F4 to enable Safe Mode, or F5 for Safe Mode with Networking (if you need internet access for updates).

macOS:

• Shut down your Mac.
• Turn it on and immediately hold down the Shift key.
• Release the key when you see the login screen.
• Log in (you may see “Safe Boot” in the top-right corner).

Once in Safe Mode, proceed with your virus scan. This environment significantly increases the chances of detecting stealthy malware.

5. Use Built-In Security Tools

Modern operating systems come with robust built-in antivirus tools. Don’t overlook them.

Windows Defender (Microsoft Defender Antivirus)

• Open the Start menu and search for “Virus & threat protection.”
• Click “Quick scan” to begin a basic scan.
• For deeper detection, click “Scan options” and select “Full scan” or “Microsoft Defender Offline scan.”
• “Offline scan” reboots your computer and scans before Windows fully loads—ideal for rootkits and persistent malware.
• Wait for the scan to complete. If threats are found, follow prompts to quarantine or remove them.

macOS: XProtect and Gatekeeper

• macOS includes XProtect, Apple’s built-in malware detection system, which automatically scans downloaded files.
• Gatekeeper prevents apps from unknown developers from running unless you explicitly allow them.
• Open “System Settings” > “Privacy & Security.”
• Check for any warnings about blocked apps or unrecognized developers.
• Use “Activity Monitor” to look for suspicious processes under the “CPU” or “Memory” tabs.

While built-in tools are effective for common threats, they may not catch advanced or zero-day malware. Use them as a first line of defense, but consider supplementing with third-party tools for comprehensive coverage.

6. Run a Third-Party Antivirus Scan

For maximum protection, use a reputable third-party antivirus or anti-malware program. These tools often have more advanced detection engines, behavioral analysis, and cloud-based intelligence than built-in solutions.

Popular options include:

• Bitdefender
• Kaspersky
• Norton
• Malwarebytes
• Avast
• ESET

How to perform a scan using Malwarebytes (example):

• Download and install Malwarebytes from its official website (malwarebytes.com).
• Launch the application.
• Click “Scan” to start a Threat Scan.
• Wait for results—this may take 10–30 minutes depending on system size.
• Review detected items. Malwarebytes will categorize them as “Malware,” “PUP” (Potentially Unwanted Program), or “Adware.”
• Select “Quarantine” or “Remove” for all detected threats.
• Restart your computer if prompted.

Always download antivirus software from the official vendor’s website. Third-party download sites often bundle malware with legitimate tools.

7. Scan for Browser Extensions and Hijackers

Many infections originate from malicious browser extensions or hijacked settings. Even if your system appears clean, your browser may be compromised.

Google Chrome:

• Click the three dots > “Extensions.”
• Review each installed extension. Remove any unfamiliar or suspicious ones.
• Go to “Settings” > “Privacy and security” > “Security” and ensure “Protect you and your device from dangerous sites” is enabled.
• Check “Settings” > “On startup” and “Default search engine” for unauthorized changes.

Mozilla Firefox:

• Click the menu button > “Add-ons and themes.”
• Under “Extensions,” disable or remove suspicious add-ons.
• Go to “Settings” > “Home and new windows” to verify your homepage.

Microsoft Edge:

• Click the three dots > “Extensions.”
• Remove unknown extensions.
• Go to “Settings” > “Startup and home page” to reset if needed.

Use tools like AdwCleaner (by Malwarebytes) to automatically detect and remove browser hijackers, toolbars, and unwanted search engines.

8. Check Startup Programs

Malware often sets itself to launch automatically when your computer starts. This ensures persistence across reboots.

Windows:

• Press Ctrl + Shift + Esc to open Task Manager.
• Go to the “Startup” tab.
• Review the list. Look for unknown or suspicious entries.
• Right-click any suspicious item and select “Disable.”

macOS:

• Go to “System Settings” > “General” > “Login Items.”
• Review the list of apps that open at login.
• Click the “-” button to remove unfamiliar items.

After disabling suspicious startup programs, reboot your computer and monitor performance. If the system runs faster and no longer exhibits odd behavior, you’ve likely removed a persistent threat.

9. Review Installed Programs

Malware often disguises itself as legitimate software. Check your list of installed programs for anything unusual.

Windows:

• Press Windows + R, type “appwiz.cpl,” and press Enter.
• Sort by “Install Date” to see recently added programs.
• Look for names you don’t recognize or programs with odd spellings (e.g., “Adobe Reader 2024” instead of “Adobe Acrobat Reader DC”).
• Uninstall suspicious programs using the “Uninstall” button.

macOS:

• Open “Finder” > “Applications.”
• Look for apps you don’t remember installing.
• Drag suspicious apps to the Trash.
• Use a tool like AppCleaner to remove associated files and preferences.

Be cautious: Some malware mimics legitimate software names. If unsure, search the program name online to verify its legitimacy.

10. Perform a Deep Scan with Multiple Tools

No single tool catches every type of malware. For a truly thorough cleanup, use multiple scanners:

1. Run a full scan with your primary antivirus (e.g., Bitdefender).
2. Run a separate scan with Malwarebytes.
3. Use HitmanPro for a second-opinion scan (it’s cloud-based and detects threats missed by local scanners).
4. Run ESET Online Scanner as a final check.

Run these tools one at a time. Running multiple scanners simultaneously can cause system conflicts or slowdowns. After each scan, reboot and check for residual symptoms.

11. Check for Rootkits

Rootkits are among the most dangerous types of malware. They hide deep within the operating system, making them invisible to standard scans.

To detect rootkits:

• Use Kaspersky TDSSKiller (free tool from Kaspersky).
• Download and run it in Safe Mode.
• Click “Scan.”
• If rootkit components are found, select “Cure” or “Delete.”
• Restart your computer.

Rootkits often require specialized tools because they operate at the kernel level. If you suspect a rootkit infection and are unable to remove it, consider backing up your data and performing a clean OS reinstall.

12. Restore System Settings and Reset Browsers

After removing malware, restore your system to a clean state:

• Reset your browser settings to default (Chrome: Settings > Reset settings; Firefox: Help > Troubleshooting Information > Refresh Firefox).
• Restore your DNS settings to automatic (Control Panel > Network and Internet > Network and Sharing Center > Change adapter settings > Right-click your connection > Properties > Internet Protocol Version 4 > Use DNS server addresses automatically).
• Use Windows System Restore (if enabled) to revert to a point before the infection occurred.

System Restore creates restore points automatically, but malware can sometimes infect them. Use this feature only if you’re confident the restore point is clean.

13. Change Passwords and Secure Accounts

If your computer was infected, assume that passwords, cookies, or login sessions may have been captured. Change passwords for:

• Email accounts
• Banking and financial services
• Cloud storage (Google Drive, iCloud, Dropbox)
• Work-related portals
• Shopping and social media accounts

Use a password manager to generate and store strong, unique passwords. Enable two-factor authentication (2FA) wherever possible to add an extra layer of security.

14. Monitor Your System After Cleanup

Even after a successful scan, continue monitoring your system for 1–2 weeks:

• Check Task Manager/Activity Monitor daily for unusual CPU or network usage.
• Watch for pop-ups, redirects, or changes to browser settings.
• Run weekly quick scans with your antivirus.
• Keep all software updated.

Reinfection is common if the source of the original infection (e.g., a malicious email, download, or USB drive) is not addressed.

Best Practices

1. Schedule Regular Scans

Don’t wait for symptoms to appear. Set up automatic scans:

• Windows Defender: Go to “Virus & threat protection” > “Manage settings” > “Virus & threat protection settings” > “Scan schedule.” Set it to weekly.
• Third-party tools: Most allow you to schedule daily, weekly, or monthly scans. Enable this feature.

Schedule scans during off-hours (e.g., late at night) to avoid performance disruption.

2. Keep Software Updated

Outdated software is the

1 cause of malware infections. Enable automatic updates for:

• Operating system
• Web browsers
• Plugins (Flash, Java, Adobe Reader)
• Office suites and productivity tools
• Antivirus software

Use tools like Patch My PC (Windows) or MacUpdate (macOS) to automate updates for third-party applications.

3. Avoid Suspicious Downloads and Email Attachments

Most malware spreads through phishing emails and malicious downloads. Follow these rules:

• Never open email attachments from unknown senders—even if they appear to come from a trusted contact (their account may be compromised).
• Verify file extensions: .exe, .bat, .scr, .js, .vbs, .msi, .zip, .rar can all contain malware. Be extra cautious with .zip files that contain executables.
• Download software only from official websites. Avoid “cracked” or “free premium” software.
• Use browser extensions like Web of Trust (WOT) or McAfee WebAdvisor to warn you about risky websites.

4. Use a Firewall

A firewall monitors incoming and outgoing network traffic. Enable the built-in firewall on your OS:

• Windows: Go to “Control Panel” > “Windows Defender Firewall” > Ensure it’s turned on.
• macOS: Go to “System Settings” > “Network” > “Firewall” > Turn it on.

For advanced users, consider a third-party firewall like GlassWire or Comodo Firewall for granular control over application network access.

5. Back Up Your Data Regularly

Malware like ransomware encrypts your files and demands payment. The best defense is regular backups.

• Use the 3-2-1 rule: 3 copies of your data, on 2 different media, with 1 copy offsite.
• Use cloud backups (iCloud, Google Drive, OneDrive) or external hard drives.
• Disconnect external drives after backup to prevent them from being encrypted in a ransomware attack.

6. Limit User Privileges

Run your computer under a standard user account—not an administrator account—for daily tasks. Malware installed under a standard account has limited access to system files.

Windows: Go to “Settings” > “Accounts” > “Family & other users” > Change account type to “Standard.”

macOS: Go to “System Settings” > “Users & Groups” > Click the lock > Right-click your account > Change to “Standard.”

7. Educate Yourself on Social Engineering

Malware often relies on tricking users. Learn to recognize:

• Urgent messages: “Your account will be suspended!”
• Too-good-to-be-true offers: “Free iPhone!”
• Impersonation: Fake tech support calls or emails pretending to be from Microsoft or Apple.

Always verify the source before clicking links or downloading files.

Tools and Resources

Free and Reputable Scanning Tools

• Malwarebytes – Excellent for detecting PUPs, adware, and ransomware. Free version allows on-demand scans.
• HitmanPro – Cloud-based second-opinion scanner. Very effective for stubborn infections.
• Kaspersky Virus Removal Tool – Free, lightweight scanner that detects and removes active threats.
• Trend Micro HouseCall – Online scanner with no installation required.
• AdwCleaner – Specialized tool for removing adware, toolbars, and browser hijackers.
• Kaspersky TDSSKiller – Dedicated rootkit scanner.
• ESET Online Scanner – Powerful cloud-based scanner that detects malware missed by local tools.
• Windows Defender Offline – Built-in tool that boots before Windows to scan for deep infections.

Monitoring and Maintenance Tools

• Process Explorer (by Microsoft Sysinternals) – Advanced task manager to analyze running processes.
• CCleaner – Cleans temporary files and manages startup programs (use cautiously; avoid registry cleaner).
• Glary Utilities – System optimization and malware cleanup tool.
• Windows Security Health – Built-in dashboard to monitor security status.
• Security Task Manager – Rates running processes for safety risk.

Online Resources

• malwarebytes.com – Comprehensive guides and free tools.
• kaspersky.com – Threat reports and removal tools.
• US-CERT – Government alerts on emerging threats.
• BleepingComputer – Community-driven malware removal forums.
• Anti-Malware.org – Educational resources on malware prevention.

Recommended Antivirus Software (Paid)

• Bitdefender Total Security – Best overall protection with low system impact.
• Kaspersky Internet Security – Excellent detection rates and parental controls.
• Norton 360 Deluxe – Includes VPN, cloud backup, and dark web monitoring.
• ESET NOD32 Antivirus – Lightweight and highly effective for advanced users.

Real Examples

Example 1: Ransomware Attack on a Small Business

A freelance graphic designer received an email with an invoice attachment labeled “Invoice_2024.pdf.exe.” Believing it to be a PDF, they opened it. The file executed a ransomware payload that encrypted all design files and displayed a message demanding $500 in Bitcoin.

Response:

• They immediately disconnected the computer from the network.
• Booted into Safe Mode with Networking.
• Used Malwarebytes and HitmanPro to scan and remove the ransomware.
• Restored files from a recent external backup.
• Changed all passwords and enabled 2FA.
• Installed Bitdefender and scheduled daily scans.

Outcome: All data recovered. No ransom paid. System restored within 48 hours.

Example 2: Browser Hijacker on a Home PC

A college student noticed their browser homepage changed to “searchmyweb.net,” and every search redirected to a fake Google page filled with ads. They tried resetting the browser, but the issue returned after reboot.

Response:

• Used AdwCleaner to scan and remove browser hijackers.
• Checked Task Manager and found a suspicious process named “SysUpdate.exe.”
• Used Malwarebytes to detect and quarantine the malware.
• Reset Chrome settings and removed unknown extensions.
• Disabled startup entries related to the hijacker.

Outcome: Browser returned to normal. No further redirects. System performance improved.

Example 3: Rootkit Infection on a Work Laptop

An IT professional noticed their Windows laptop was running extremely slow, even after a fresh OS install. Antivirus scans showed no threats, but the issue persisted.

Response:

• Booted into Safe Mode and ran Kaspersky TDSSKiller.
• The tool detected a rootkit named “TDL4” embedded in the Master Boot Record (MBR).
• Used TDSSKiller to remove the rootkit.
• Repaired the MBR using Windows Recovery Environment.
• Performed a full system scan with ESET and Windows Defender.

Outcome: System fully cleaned. Rootkit eliminated. No recurrence after 6 months.

FAQs

How often should I scan my computer for viruses?

Perform a quick scan weekly and a full scan monthly. If you frequently download files, visit risky websites, or use public Wi-Fi, consider scanning every few days. Enable real-time protection and automatic updates for continuous defense.

Can Windows Defender remove all viruses?

Windows Defender is effective against common threats and has improved significantly. However, it may miss advanced, zero-day, or fileless malware. For comprehensive protection, use it alongside a third-party scanner like Malwarebytes for periodic deep scans.

Do I need antivirus software on macOS?

While macOS has built-in protections, it’s not immune. Mac-targeted malware is increasing. Use a reputable antivirus tool (e.g., Bitdefender, Malwarebytes) for added security, especially if you share files with Windows users or download software from unofficial sources.

What should I do if a virus can’t be removed?

If malware resists removal:

• Boot into Safe Mode and try again.
• Use multiple scanners (e.g., Malwarebytes + HitmanPro).
• Check for rootkits using specialized tools.
• If all else fails, back up your personal files (scan them first!) and perform a clean OS reinstall.

Can viruses spread through USB drives?

Yes. Malware can auto-execute when a USB drive is inserted. Always scan USB drives with antivirus software before opening files. Disable AutoRun in Windows (Control Panel > AutoPlay) to prevent automatic execution.

Is it safe to use free antivirus software?

Yes, if you use reputable tools like Malwarebytes Free, Bitdefender Free, or Windows Defender. Avoid fake or unverified “free antivirus” programs from unknown websites—they often contain malware themselves.

How do I know if a scan is legitimate?

Legitimate antivirus tools are downloaded from official websites. Pop-ups claiming your computer is infected and urging you to call a number are scams. Never trust unsolicited security alerts in your browser.

Can malware survive a factory reset?

In rare cases, firmware-level malware (e.g., UEFI rootkits) can persist. A factory reset typically removes software-based malware. For maximum security, use a clean OS installation from official media and avoid restoring unscanned backups.

Should I scan my computer if I didn’t download anything suspicious?

Yes. Malware can spread through compromised websites, email links, or even legitimate software updates. Regular scanning is a preventive measure, not just a reactive one.

Conclusion

Knowing how to scan computer for viruses is not a one-time task—it’s an ongoing discipline essential for digital security. From recognizing early warning signs to executing multi-tool scans and adopting long-term protective habits, every step you take reduces your risk of infection. Modern malware is sophisticated, but so are the tools and strategies available to combat it.

By following this guide, you’ve equipped yourself with the knowledge to detect, remove, and prevent viruses effectively. Remember: prevention is always better than cure. Keep your systems updated, avoid risky downloads, back up your data, and scan regularly. The combination of awareness, the right tools, and consistent habits forms an impenetrable shield against digital threats.

Your computer is more than a device—it’s a gateway to your personal and professional life. Protect it with diligence, and you’ll enjoy peace of mind in an increasingly connected world.