How to Scan for Viruses on Windows 11 and Windows 10 (and When to Use Malwarebytes)

You don't need to buy anything to scan a Windows PC for viruses — Microsoft Defender is built into Windows 10 and 11, it's free, and it scores at or near the top in independent lab tests. For the stubborn stuff Defender occasionally leaves behind — adware, browser hijackers, and "potentially unwanted programs" — a free second-opinion scanner like Malwarebytes is the tool we reach for. Here's how to run both.

Scan with the built-in Microsoft Defender (free)

Open Windows Security: click Start, type "Windows Security," and open it. (You can also get there via Settings — on Windows 11: Privacy & security → Windows Security; on Windows 10: Update & Security → Windows Security.)

Click "Virus & threat protection," then "Quick scan" for a fast check of the usual hiding spots. For a thorough sweep of the whole drive, click "Scan options" and choose "Full scan" (it takes a while — let it run).

Make sure real-time protection is on and definitions update automatically (they do by default) — that's what stops most threats before a scan is ever needed.

For stubborn infections: the offline scan

If a PC is acting infected but a normal scan comes back clean — or won't finish — use the offline scan. Under "Scan options," choose "Microsoft Defender Antivirus (offline scan)." The PC reboots and scans before Windows fully loads, which catches deeply embedded malware (rootkits) that can hide while Windows is running. It takes about 15 minutes and the computer restarts on its own — save your work first.

Get a second opinion with Malwarebytes

Defender is strong, but a dedicated on-demand scanner catches a different slice of junk — adware, sketchy browser extensions, bundled "free" programs, and the pop-up/scareware ecosystem. Malwarebytes is the one we recommend for that second opinion.

Important: download it ONLY from the official site, malwarebytes.com — never from a pop-up or an ad telling you that you're infected (those are the malware). Install it, run a Scan, review what it found, Quarantine it, and reboot. The free version does manual scans, which is all you need for a second opinion; Premium adds always-on protection, but you don't need that running alongside Defender unless you want it.

When to scan — and the trap to avoid

Run a scan if you see constant pop-ups, your browser's homepage or search engine changed on its own, the machine suddenly got slow, or you clicked something you regret. The one thing NOT to do: if a full-screen page or pop-up says "Your computer is infected — call this number" or "paste this command to fix it," that IS the scam. Don't call, don't paste anything, don't install what it tells you to — just close the tab (or restart) and run your own Defender or Malwarebytes scan instead.

Other reputable, free, official on-demand scanners if you want a third look: Microsoft Safety Scanner and ESET Online Scanner.

If it won't come clean

Some infections fight back — they block the scanner, hijack the browser, or keep coming back after a reboot. If a scan can't finish or the problem returns, that's our cue. We remove malware safely (without wiping your files unless it's truly the only option), restore your browser and email, and harden the machine so it doesn't happen again — onsite or remotely.