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On Windows 10 or 11, the best place to start is the antivirus you already have: the Windows Security app. Run a Quick scan first. If you’re still worried, update protection and run a Full scan. If malware keeps coming back, run Microsoft Defender Offline, which scans outside your normal Windows session. This guide covers Windows only.
The step-by-step check
1. Open Windows Security and look at the current status
Open Windows Security and select Virus & threat protection. Check whether any current threats are listed and when the last scan ran. If you’re worried about one download or folder, right-click it in File Explorer and choose Scan with Microsoft Defender. On Windows 11, Microsoft says this command may sit under Show more options in the context menu.
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2. Run a Quick scan
Select Virus & threat protection > Quick scan. Microsoft Support’s “Stay protected with the Windows Security app” page says: “If you suspect there’s malware or a virus on your device, you should immediately run a quick scan.”
3. Update protection before going deeper
Go to Virus & threat protection > Protection updates and check for updates. Microsoft says these security intelligence definitions are used every time a scan runs, so an outdated set weakens the result. Microsoft’s troubleshooting guidance also recommends turning on cloud-delivered protection and automatic sample submission. Both help catch malware that hasn’t been seen before.
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4. Run a Full scan if concern remains
Choose Scan options > Full scan. Per Microsoft’s “Virus and Threat Protection in the Windows Security App” page, it “scans every file and program on your device.” It can take a long time on big drives or with many large files and archives. Close unneeded apps and let it finish.
5. Use Microsoft Defender Offline for stubborn or recurring threats
- Save your open work.
- Go to Scan options > Microsoft Defender Offline scan > Scan now.
- The PC restarts and scans in the Windows Recovery Environment, without loading your usual Windows session. That helps with malware that hides or reinstalls itself while Windows is running.
- When it finishes, Windows restarts. Review the outcome in Protection history.
6. Reinstall Windows if the infection continues
Microsoft’s advice is to try Windows Security scanning first. If the problem persists, do a clean installation from installation media. Back up the files you need first. If you have an external backup made before the infection, prefer restoring from that. You can create the installation media on a USB drive from another working PC.
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Which scan to choose
| Option | What it checks | Restart needed? | Use it when |
|---|---|---|---|
| Quick scan | Common locations, quickly | No | First check when you suspect malware |
| Full scan | Every file and program on the device | No | Quick scan didn’t settle your concern, or you want broad coverage |
| Custom scan | Files and folders you select | No | You want to check one location |
| Microsoft Defender Offline | Scan run in the Windows Recovery Environment, outside the normal session | Yes | Malware recurs or persists, or you want to scan without an internet connection |
Clean reinstallation is the step beyond all of these. It is a recovery action, not a scan.
Things that can undermine a scan
- Another antivirus is installed. Microsoft Defender Antivirus may switch to disabled mode automatically. Check Virus & threat protection > Who’s protecting me? > Manage providers to see which provider is active. Microsoft says Defender should return to active mode once the other product is uninstalled.
- Broad exclusions. Don’t add them just to speed up scanning. Microsoft warns that excluded files and folders are no longer checked by real-time protection.
- Low disk space. Microsoft notes that available disk space can affect malware removal.
If a threat is found
Follow the action Windows Security offers, such as quarantine or removal, then rescan. Microsoft’s Defender overview says it may block, notify and try to remove threats automatically, but sometimes asks you to act. If the same threat returns, move on to the offline scan.
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What a clean result means
A clean scan shows that Microsoft’s scanner found nothing. It is not proof the PC is malware-free, because Microsoft describes these tools and escalation steps, not a guarantee of detection. If the PC still behaves suspiciously after a clean result, the offline scan and then a clean reinstall are the next steps.
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