A slow, misbehaving PC doesn't always mean a virus, but when pop-ups appear unprompted, programs crash constantly, or your antivirus starts throwing alerts, it's time to run a proper, thorough scan. This guide walks through exactly how to check your PC for viruses using free, trustworthy tools, what to do once something is found, and how to reduce the chances of reinfection.

Signs you might have a virus

  • Unusually slow performance, especially high disk or CPU usage with no obvious cause
  • Programs crashing or freezing frequently
  • Pop-up adverts appearing even when your browser is closed
  • Your antivirus software has been disabled without your knowledge
  • New toolbars, extensions, or a changed browser homepage you didn't set
  • Friends receiving strange messages or emails from your accounts that you didn't send
  • Unusually high data usage or your fan running constantly at full speed

For a fuller list of red flags and how to interpret them, see our companion article on signs your computer is infected with malware.

Step 1: Disconnect from the internet (if you suspect active infection)

If you strongly suspect an active infection — particularly if you're seeing signs of data being sent out, ransom messages, or unusual network activity — disconnect from Wi-Fi or unplug the Ethernet cable before scanning. This prevents malware from communicating with its control server or spreading further, and stops it exfiltrating any personal data while you clean up.

For routine "I want to check my PC is clean" scans, you can skip this step and stay connected, since your antivirus needs an internet connection to download the latest virus definitions anyway.

Step 2: Update Windows and your antivirus definitions

An outdated antivirus is far less effective, since new threats appear daily.

  1. Go to Settings > Windows Update and install any pending updates, then restart if prompted.
  2. Open Windows Security (search for it in the Start menu) and go to Virus & threat protection.
  3. Click Check for updates under Virus & threat protection updates to ensure your definitions are current.

Step 3: Run a full scan with Windows Defender

Windows Defender (now called Windows Security) is built into every modern version of Windows and is a genuinely capable antivirus for most everyday threats.

  1. Open Windows Security from the Start menu.
  2. Go to Virus & threat protection.
  3. Click Scan options.
  4. Select Full scan rather than Quick scan — this checks every file on your drives, not just common infection points, and can take an hour or more depending on how much data you have.
  5. Click Scan now and let it run to completion without using the PC heavily in the meantime.
  6. If threats are found, Windows Defender will list them and offer to Remove, Quarantine, or Allow each one — choose Remove or Quarantine unless you're certain a flagged file is a false positive.

Pro tip: If you suspect deep-rooted malware that's actively hiding itself, use Windows Security > Virus & threat protection > Scan options > Microsoft Defender Offline scan. This restarts your PC and scans before Windows fully loads, which catches malware designed to hide from scans run within a live Windows session.

Step 4: Run a second opinion scan with Malwarebytes

No single antivirus catches everything, so running a second, different tool as a "second opinion" scan is good practice, especially if Defender found nothing but symptoms persist.

  1. Download Malwarebytes free edition from the official malwarebytes.com website (never from a third-party download site or a link in an email).
  2. Install it and open the app.
  3. Click Scan to run its standard scan, which checks for malware, adware, and potentially unwanted programs (PUPs) that traditional antivirus tools sometimes miss.
  4. Once complete, review the list of detected items and click Quarantine Selected to remove them.
  5. Restart your PC if prompted.

Warning: Be very careful when searching for antivirus tools online — malicious adverts and fake download sites frequently impersonate legitimate antivirus brands. Always type the official website address directly rather than clicking search results or ads, and verify the download is signed by the genuine publisher.

Step 5: Check installed programs for anything unfamiliar

Some malware installs itself as a seemingly legitimate program rather than hiding as a hidden file.

  1. Go to Settings > Apps > Installed apps.
  2. Sort by Install date if available, and look through recently installed items.
  3. Research any unfamiliar program names before removing them — a quick web search on another device usually reveals quickly whether something is a known nuisance program.
  4. Uninstall anything suspicious via the three-dot menu next to it, then Uninstall.

Step 6: Check your browser for unwanted extensions

Malware and adware frequently hijack browsers by installing unwanted extensions. In Chrome, go to Settings > Extensions; in Edge, Settings > Extensions; in Firefox, Add-ons and themes. Remove anything you don't recognise or didn't install deliberately. For a deeper walkthrough of this specific problem, see our guide on removing adware and browser hijackers.

Step 7: Reset your browser settings

If your homepage, search engine, or new tab page has been changed without your permission, reset the browser to its defaults after removing any malicious extensions:

  • Chrome: Settings > Reset settings > Restore settings to their original defaults
  • Edge: Settings > Reset settings > Restore settings to their default values
  • Firefox: Help > More troubleshooting information > Refresh Firefox

Step 8: Change your important passwords

If there's any chance malware was capturing keystrokes or browser data (common with trojans and info-stealers), change the passwords for your most important accounts — email, online banking, and any account reusing a password — from a different, known-clean device once your PC is confirmed clean. Enable two-factor authentication wherever it's offered, as this protects you even if a password was captured.

Step 9: Consider a clean reinstall for severe infections

If scans keep finding the same infection repeatedly, if your PC exhibits rootkit-like behaviour (antivirus tools being silently disabled, Task Manager not opening), or if you've experienced ransomware, the safest option is a clean reinstall of Windows.

  1. Back up your important personal files (documents, photos) to an external drive — but scan them first, and be cautious about backing up executable (.exe) files.
  2. Go to Settings > System > Recovery and choose Reset this PC, selecting Remove everything for a thorough clean install.
  3. Reinstall your applications from official sources afterwards, and restore only your personal files, not full system backups, to avoid reintroducing the infection.

For a full walkthrough of recovering from the most severe case — ransomware — see our dedicated article on recovering from a ransomware attack.

Preventing future infections

  • Keep Windows Update and your antivirus definitions on automatic updates.
  • Avoid downloading software from unofficial sites, cracked software, or torrents — this remains one of the most common infection routes.
  • Be cautious with email attachments and links, even from contacts, since accounts can be spoofed or compromised.
  • Use a standard (non-administrator) user account for daily use where practical, which limits what malware can do if it does get in.
  • Keep regular backups of important files on an external drive or cloud service, disconnected from your PC when not in use.

Closing thoughts

Running a full scan with both Windows Defender and Malwarebytes catches the vast majority of infections on a typical home PC, and following up by checking installed programs, browser extensions, and changing passwords closes the loop on anything that slipped through. If symptoms persist after thorough scanning, or if you're dealing with ransomware or rootkit-level behaviour, a clean Windows reinstall is the most reliable way to guarantee a fresh start.

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