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If you suspect malware on a Windows PC, update Microsoft Defender’s security intelligence and run a full scan. Review any detection and choose an appropriate action; do not allow a file unless you trust it. If malware persists or returns, save your work and run Microsoft Defender Offline. Consider recovery or a clean Windows installation only if scanning does not solve the problem or Windows will not start.
Could the symptoms mean malware?
Unexpected pop-ups, browser redirects, a slow PC, or unusual data use can be signs of malware, but they do not prove an infection. Other software or system problems can cause similar behavior. Scan the PC rather than treating one symptom as a diagnosis.
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Microsoft’s recommended first step is a virus scan with Windows Security. Its Recovery options in Windows page states: “If you suspect your device has been infected, first try running a virus scan using Windows Security (Settings > Privacy & security > Windows Security).” Menu labels can vary by Windows version.
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What should you do before scanning?
- Keep Microsoft Defender real-time protection enabled. Turning it off leaves files you open or download unscanned; adding an exclusion means Defender will not check the excluded item.
- Avoid running multiple real-time antivirus products at once. Microsoft warns that doing so can affect performance and stability.
- Do not delete or allow a detection impulsively. Review its name and details in Windows Security before choosing an action.
How to update Defender and run a full scan
- Open Windows Security. In Windows 11, you can usually find it under Settings > Privacy & security > Windows Security; Windows 10 menus may differ.
- Open Virus & threat protection and check for security intelligence updates. Install available updates before scanning so Defender has current threat information.
- Choose Scan options, select Full scan, and start the scan. A full scan checks every file and program on the device, so it can take longer and may slow the PC while it runs.
- Wait for the scan to finish, then review the detection details and results in Windows Security.
A quick scan is a faster check of locations where threats are commonly found; it is useful for a routine check, but Microsoft recommends a full scan when you think the PC is infected. If you are concerned about one particular file or folder, right-click it in File Explorer and choose the custom scan option if it is available in your Windows version.
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What should you do if Defender finds a threat?
Open the threat details and Protection history in Windows Security, then choose an action appropriate to the detection. These options have different effects:
- Quarantine: Moves the detected file to a safe location and blocks it from running.
- Remove: Deletes the detected file.
- Allow: Permits the file to run and suppresses later alerts for it. Use this only if you trust the file and its publisher.
If a result says it was only partially removed, some detected malware files may remain. Follow Microsoft’s further troubleshooting guidance, which includes additional scanning and may include an offline scan. Do not assume that a detection is harmless simply because Windows has taken some action.
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When should you run Microsoft Defender Offline?
Use an offline scan if malware keeps returning, a normal scan cannot resolve the issue, or you are concerned that malicious software may interfere with a scan in Windows. Defender Offline restarts the PC and scans from the Windows Recovery Environment without loading normal Windows, making it harder for persistent malware to hide.
- Save open work and close your apps.
- In Windows Security, open Virus & threat protection > Scan options and select Microsoft Defender Offline scan.
- Start the scan and allow the PC to restart. It scans in the recovery environment and restarts again when finished.
- After Windows returns, open Protection history to review the result and any action taken.
What if scans do not fix the problem or Windows will not start?
Consider Windows recovery only after scanning has failed to resolve the issue, or if Windows cannot start. Recovery choices include System Restore, resetting Windows, or reinstalling it; their effects differ, so check the options for your Windows version before proceeding.
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Back up before a reset or reinstall
Save files you still need before using a recovery option that may remove data. If restoring files after cleanup, prefer a backup made before the infection and stored externally. No backup or restored file should be assumed safe without qualification.
Use installation media only when necessary
Microsoft says a clean installation from Windows installation media may help remove malware from the Windows installation if the issue continues. It is a disruptive last resort: it removes Windows, personal files, apps, and settings from the selected drive. If the PC will not start and its recovery options fail, you can create installation media using another working PC. A manufacturer recovery image may include drivers and factory applications intended for that hardware. If the computer is managed by work or school, contact the IT team before attempting recovery.
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How can you reduce the chance of another infection?
- Keep Windows, your browser, and apps updated.
- Leave real-time protection enabled and avoid broad Defender exclusions.
- Be cautious with downloads and unexpected email attachments.
- Use protections such as Windows 11 Smart App Control and Microsoft Edge SmartScreen where available, but do not treat any single feature as a guarantee against malware.
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