How to Run a Malware Scan on Windows
Worried your PC has picked something up? The good news is you don't need to download anything — modern Windows includes a genuinely capable scanner, Microsoft Defender, built right in. In fact, reaching for a random "free virus removal" tool from a pop-up is one of the quickest ways to make a healthy PC unhealthy. This guide shows you how to run a proper check using the built-in tools, what to do if it finds something, and how to tell a real infection from an innocent slowdown.
First, is it actually malware?
Before scanning, it's worth a reality check: a slow computer is far more often a tired one than an infected one — a full hard drive, too little memory, or an ageing mechanical drive (see our guide to a computer running slow). The signs that genuinely point to malware are things like unexpected pop-ups and adverts, your browser's home page or search engine changing on its own, programs you don't recognise, or your security software being switched off. Spot a few of those together and a scan is absolutely the right move.
1. Open Windows Security
Click the Start menu, type "Windows Security" and open it. This is the home for Microsoft Defender, which runs continuously in the background but can also scan on demand.
2. Go to Virus & threat protection
Select "Virus & threat protection" from the menu. Here you'll see the results of recent scans and the option to run a new one.
3. Run a Quick scan
Click "Quick scan". This checks the folders and processes where malware most commonly hides and takes only a few minutes — a sensible first pass that catches the majority of everyday threats.
4. Run a Full scan if needed
If the quick scan finds something, or you're still suspicious, go to "Scan options" → "Full scan". This checks every file on the drive, so it's much slower (often an hour or more) but far more thorough. It's worth starting one before you step away from the computer for a while.
5. Use an Offline scan for stubborn threats
Some of the nastiest malware hides while Windows is running, making it hard to remove. "Microsoft Defender Offline scan" restarts your PC and scans before Windows fully loads, catching things a normal scan can't. The computer will reboot on its own, run the scan, and return you to the desktop when it's done — so save your work first.
What to do if it finds something
If a scan detects threats, let Windows Security quarantine or remove them, then restart and run another scan to confirm it's clean. Change the passwords for any important accounts — ideally from a different, trusted device — and turn on two-factor authentication if you haven't already, in case anything was captured. Keep an eye out over the next few days for the warning signs returning.
A serious word of caution on "cleaner" tools
Stick to Microsoft Defender or one well-known, reputable security product. The "free PC cleaner", "speed booster" and "virus removal" tools advertised in pop-ups and dodgy download sites are frequently the actual problem — they bundle adware, nag you to pay for fake fixes, or are outright malware themselves. The same goes for any phone call or pop-up telling you to ring a number because you're "infected": that's a classic tech-support scam, not a real warning. Genuine security alerts never ask you to call someone. For the wider picture on whether you need anything beyond the built-in tools, see do you still need antivirus?
How often should you scan?
Here's the reassuring part: Microsoft Defender scans continuously in the background, so for most people there's no need to run manual scans on a schedule — it's already watching every file as it opens. Run a manual quick scan when you actually notice something off, after installing software from anywhere you're not completely sure about, or if a website threw up a warning. The bigger protection, though, isn't more scanning at all — it's good habits: keep Windows updated, use strong passwords and 2FA, and think before clicking links or opening attachments. Antivirus is the safety net; your own caution is the first line of defence, and it's the one that stops most infections ever reaching the scanner.
Can't shift it?
Some infections hide, reinstall themselves, or leave back doors open, so if a scan keeps finding things, the PC's still misbehaving, or you're simply not confident it's truly gone, don't keep guessing. Our computer repair and cyber security teams remove malware thoroughly, check what it touched, and close the gap it came in through — and where there's any doubt, a clean install is the guaranteed fix. Watch for the full list of warning signs of infection too.
Official guidance: For official guidance on keeping devices free of malware, see the UK’s NCSC Cyber Aware campaign.