The fastest way to check for viruses on your PC

Run a full system scan using Windows Defender, which comes built into Windows 10 and 11 and requires no installation. Open Windows Security (search for it in the Start menu), select Virus & threat protection, then click Scan options and choose Full scan. The scan takes 30 minutes to several hours depending on how many files you have, but it checks every file on your computer, not just obvious danger zones.

If you prefer a second opinion or suspect Windows Defender missed something, Malwarebytes is a free alternative that catches threats Defender sometimes overlooks. Download it from malwarebytes.com, install it, and run a full scan the same way. Many people run both — Defender first, then Malwarebytes — because they use different detection methods and catch different things.

Do not rely on quick scans. A quick scan checks only the places malware usually hides (startup folders, temporary files, browser extensions) and takes 5 to 10 minutes, but it misses infections buried deeper in your system. Use a full scan if you suspect an actual problem.

Key Takeaways

  • Windows Defender, built into Windows 10 and 11, runs a full system scan in 30 minutes to several hours and catches most common threats.
  • A full scan checks every file on your computer, while a quick scan checks only common hiding places and misses deeper infections.
  • Malwarebytes is a free second-opinion tool that catches threats Defender sometimes overlooks and works alongside it.
  • Close other programs before scanning, disable your VPN temporarily if you have one, and let the scan finish without interruption.
  • If a scan finds threats, the program will quarantine them automatically, but you should restart your computer afterward to make sure the removal sticks.

What to do before you start scanning

Close all open programs and browser windows. Scans run faster and more reliably when your computer is not doing anything else. If you have a VPN running, turn it off temporarily — some security software conflicts with VPNs and produces false results.

Plug in your power adapter if you are on a laptop. A full scan drains battery quickly, and if your computer goes to sleep mid-scan, you may have to start over. Set your screen to never sleep while the scan runs (you can change this back afterward in Settings > System > Power & sleep).

Do not restart your computer during the scan. If Windows forces an update restart, let the scan finish first if possible, or restart and run it again. Interrupting a scan leaves it incomplete and wastes time.

Understanding what happens when threats are found

When a scan finds a threat, the security program automatically moves it to quarantine — a locked folder where the malware cannot run or spread. You do not have to do anything during the scan. Once it finishes, the program shows you a summary of what it found and where it sent the threats.

Quarantine is safe. The threat is isolated and cannot harm your computer. However, quarantine is not permanent removal — the file still exists on your hard drive, just locked away. Most people leave quarantined files alone, but if you want to permanently delete them, the security program will let you do that from the quarantine menu.

After a scan finds and quarantines threats, restart your computer. This clears temporary files and ensures that any malware still running in your computer's memory is stopped. Some infections hide in memory and only fully disappear after a restart.

When to use Windows Defender versus third-party tools

Windows Defender is enough for most people. It runs automatically in the background, updates itself, and catches the majority of common viruses and malware. You do not pay for it, and it does not slow your computer down noticeably.

Use a third-party tool like Malwarebytes if you suspect a serious infection, if your computer is running unusually slowly, or if you want a second opinion after Defender's scan. Malwarebytes is free for basic scanning, though it offers a paid version with real-time protection. Running both tools together gives you more thorough coverage because they use different detection methods.

Avoid installing multiple paid antivirus programs at the same time. Two paid security tools conflict with each other and actually make your computer less secure. If you want to switch from one to another, uninstall the first one completely before installing the second.

Scanning specific parts of your computer

If a full scan takes too long or you want to check only the places where malware usually hides, you can scan individual folders. Right-click any folder in File Explorer, select Scan with Windows Defender, and it checks just that folder and everything inside it. This is useful if you suspect a specific download or USB drive is infected.

Browser extensions are a common hiding place for malware. In Windows Defender, go to Virus & threat protection > Manage settings and scroll down to Exclusions to see which folders Defender is not scanning — some programs add themselves to this list to avoid detection. If you see something unfamiliar, remove it from the exclusions list and run a scan.

Startup programs can also hide malware. Press Ctrl + Shift + Esc to open Task Manager, click the Startup tab, and look for programs you do not recognize. Right-click anything suspicious and select Disable. This does not remove the program, just stops it from running when your computer starts.

What to do if a scan finds nothing but your computer still seems infected

If your computer is slow, crashes often, or shows pop-ups even after a clean scan, the problem might not be malware. Slow performance usually comes from too many startup programs, a full hard drive, or an aging computer that needs more RAM. Pop-ups might be from a browser extension rather than a virus.

Check your browser extensions first. In Chrome, click the three-dot menu, go to Extensions, and look for anything you did not install yourself. In Firefox, click the menu button, select Add-ons and extensions, and do the same. Delete anything unfamiliar. Many malware programs hide as browser extensions rather than system-wide infections.

If you still suspect an infection after scanning with both Windows Defender and Malwarebytes, try a specialized tool like Kaspersky Rescue Disk, which boots your computer from a USB drive and scans before Windows even loads. This catches rootkits and other deep infections that regular scans might miss. Instructions for creating and using a Rescue Disk are on Kaspersky's website.

How often you should scan for viruses

Windows Defender runs automatic scans in the background on a schedule you can set. Most people do not need to manually scan more than once a month, unless they download files frequently from untrusted sources or visit risky websites. If you do either of those things, scan weekly or after each risky download.

Real-time protection is more important than scheduled scans. Windows Defender watches your computer constantly and stops threats the moment they arrive, before they can spread. As long as real-time protection is on (check Virus & threat protection > Manage settings to confirm), you are protected between scans.

After you scan and remove threats, keep your operating system and software updated. Most malware enters through security holes in outdated programs. Turn on automatic updates in Windows Settings, and let your browser update itself automatically too.

Frequently Asked Questions

Can I scan my computer while using it, or do I have to stop everything?

You can use your computer during a scan, but it will run much slower. Scanning uses a lot of processing power, so other programs will lag. It is better to start a scan and then leave your computer alone, or run it overnight when you are not using it.

What if Windows Defender says it found a threat but will not remove it?

Some threats require administrator permission to remove, or they are locked by another program. Restart your computer in Safe Mode (hold Shift while clicking Restart in the Start menu, then select Troubleshoot > Advanced options > Startup Settings), then run the scan again. Safe Mode stops most programs from running, so the threat cannot protect itself.

Is it safe to download and run multiple antivirus programs to scan at the same time?

Yes, you can run multiple free scanning tools like Windows Defender and Malwarebytes together. But do not install multiple paid antivirus programs that run in the background at the same time — they conflict with each other and make your computer less secure. Use one paid program for real-time protection and others only for occasional scans.

How long does a full scan usually take?

A full scan typically takes 30 minutes to two hours on a modern computer with a solid-state drive, or two to four hours on an older computer with a traditional hard drive. The time depends on how many files you have and how fast your storage is. Larger hard drives take longer to scan.

What should I do if a scan finds a threat in a program I use every day?

Quarantine it first and see if your computer works fine without it. If the program stops working, you may need to reinstall it from the official website. If the threat keeps coming back in the same program, that program is either compromised or flagged incorrectly — uninstall it and download a fresh copy from the developer's official site.