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First, confirm which product and Windows version you use
These steps concern the built-in Microsoft Defender Antivirus, found in the Windows Security app—not the separate Microsoft Defender subscription app. Windows 10 and Windows 11 may have different interface layouts, so check your Windows version if a setting does not appear where expected.
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Windows 10 support ended on October 14, 2025. Microsoft says it no longer provides free Windows Update software updates, technical assistance, or security fixes for Windows 10 after that date. See Microsoft’s Windows security troubleshooter guidance for the lifecycle notice. Its downloadable troubleshooter is specifically for Windows 10 and refers to the Microsoft Support Diagnostic Tool, which Microsoft says is deprecated and will be removed from future Windows versions; it is not a general repair tool for every current Windows installation.
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Open Windows Security > Virus & threat protection and check the provider status, sometimes shown under Who’s protecting me? or Manage providers. Microsoft says a compatible third-party antivirus turns Microsoft Defender Antivirus off automatically. If you recently installed or still use another antivirus, this may be expected behavior rather than a Defender failure. Microsoft says Defender should return to active mode after the other product is uninstalled. See Microsoft’s Windows Security guidance.
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If Defender is the active provider, open Windows Security > Virus & threat protection > Manage settings and check Real-time protection. This setting monitors files and programs as they are accessed or run. When it is off, files opened or downloaded during that time are not checked in real time; Microsoft says protection turns itself back on after a short time. Tamper protection can prevent other apps from changing security settings, so an attempted change may not take effect.
Update security intelligence if scans may be out of date
Security intelligence contains information about threats that Defender uses during scans. Windows Update downloads it automatically, but you can check manually: open Windows Security > Virus & threat protection > Protection updates, then select Check for updates. Microsoft’s Windows Security instructions describe this update path.
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If the check fails, note any error code and whether Windows Update itself is working. The general guidance here does not diagnose a particular update error; the exact code and Windows version matter for that problem.
Choose a scan that matches the concern
You do not need to begin with the most disruptive scan for every issue. Use the scan scope that fits what you observed:
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| Scan | What it checks | When it may fit |
|---|---|---|
| Quick scan | A rapid check of common locations where threats may be found. | As an immediate check when you have a general concern. |
| Full scan | Every file and program on the device. | When you want a broader check of the whole device. |
| Custom scan | Files or folders you select. | When you want to inspect a particular item or location. |
| Microsoft Defender Offline | Scans after restarting into Windows Recovery Environment. | When persistent malware may be able to hide or defend itself during a normal Windows session. |
Find scan choices under Windows Security > Virus & threat protection. Save open work before starting Microsoft Defender Offline because it restarts the PC. Scan results appear in Protection history. Microsoft’s scan guidance explains the available options.
Review a detection before allowing or removing a file
Open Windows Security > Virus & threat protection > Protection history to review actions Defender has taken, including detections and services turned off. Events remain in the history for two weeks, according to Microsoft’s Protection history guidance. An administrator is required to view threat details.
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If you are unsure whether a detected item is safe, Microsoft advises quarantining it. Quarantine renders the item harmless while you decide what to do. Do not allow a file simply because you recognize its name; a detection can involve a file that has been altered or used in an unexpected way. If you believe a file is safe, Microsoft provides a false-positive reporting route through its security intelligence file submission portal.
Use exclusions only for a specific, justified need
An exclusion can tell Defender not to check a file, folder, file type, or process during real-time scanning. Microsoft warns that excluded items can leave the device and its data vulnerable. Scheduled scans or another antimalware product may still scan them, so an exclusion does not necessarily stop every scan. If you must create one, make it as narrow as possible and remove it when it is no longer needed. Avoid disabling antivirus protection broadly to get a single file to run. See Microsoft’s exclusion guidance.
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When these checks do not resolve the problem
A blank Windows Security window, a scan that will not start, or an update that repeatedly fails can have different causes. The symptom alone does not establish one universal fix. Record the Windows version, the exact error message or code, whether another antivirus is installed, and which step fails. That information helps distinguish a provider-status issue from a Windows Security app, update, or device-management problem.
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