First check whether Microsoft Defender Antivirus is actually off or whether another antivirus is protecting your PC. Open Windows Security > Virus & threat protection > Manage providers and check which provider is active. If Defender is the intended provider, turn on Real-time protection, check for security intelligence updates, and scan the device. The right next step depends on the status Windows Security shows and whether the PC is managed by work or school.
1. Check which antivirus provider is active
Open Windows Security, select Virus & threat protection, then choose Manage providers under “Who’s protecting me?” This shows whether Microsoft Defender Antivirus or another security product is currently providing antivirus protection. Microsoft explains that a compatible third-party antivirus can automatically turn Defender off while that product is active. Microsoft: Virus and threat protection in the Windows Security app
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- A different antivirus is active: Defender may be off by design rather than malfunctioning. If you want to use that product, check its own status and updates.
- You want to use Defender instead: Uninstall the other antivirus from Start > Settings > Apps. Restart if prompted, then return to Manage providers and check whether Defender is active. Microsoft says Defender should return to active mode after removal; some products may not uninstall completely, in which case use the publisher’s removal instructions or contact its support. Microsoft: Scan an item with Windows Security Microsoft: Protect my PC from viruses
- No provider appears active, or Windows Security shows an error: Note the exact message and continue with the checks below. Don’t assume that installing another antivirus is the fix.
Avoid running two antivirus products together. Microsoft warns that multiple security tools can cause problems; remove an unwanted product fully before relying on Defender.
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2. Turn on Real-time protection
If Defender is the provider you intend to use, open Windows Security > Virus & threat protection > Manage settings and switch Real-time protection on. In some Windows 10 releases, the route into this page is Start > Settings > Update & Security > Windows Security > Virus & threat protection. Labels and navigation can vary slightly by release. Microsoft: Help protect my device with Windows Security
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If the setting is unavailable, switches back off, or produces an alert, record the exact wording rather than trying registry, service, or policy changes at random. A work- or school-managed PC may have protection settings controlled by an administrator.
3. Check security intelligence and Windows updates
Security intelligence—also called definitions—helps Defender identify current threats. In Windows Security, open Virus & threat protection > Protection updates > Check for updates. Microsoft also delivers security intelligence through Windows Update. Check for pending Windows updates and restart if Windows asks you to. Then revisit Manage providers and the Real-time protection setting to see whether the status has changed. Microsoft: Virus and threat protection in the Windows Security app
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There is an important Windows 10 limitation: Microsoft says support ended on October 14, 2025. After that date, Microsoft no longer provides free Windows Update software updates, technical assistance, or security fixes for Windows 10. Windows Security may still offer the update controls described above, but you should not assume an unsupported Windows 10 installation will receive new operating-system security fixes. Microsoft recommends moving to Windows 11. Microsoft: Automatically fix Windows security issues
4. Run a scan once protection is available
In Windows Security > Virus & threat protection, start a quick scan for a check of common locations, or choose scan options and run a full scan. If you suspect persistent malware is interfering with normal protection, Microsoft Defender Offline can scan outside the ordinary Windows session by restarting into the Windows Recovery Environment. Save open files before starting: the PC restarts for the offline scan. Microsoft: Virus and threat protection in the Windows Security app
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5. If Defender still will not turn on
Gather the details that can distinguish a provider conflict from a Windows Security or management issue:
- The exact alert or error text shown in Windows Security.
- Your Windows 10 version and build, available under Start > Settings > System > About.
- Whether the device is personally owned or managed by work or school.
- The names of antivirus products recently installed or removed, and whether their uninstall completed normally.
Contact your organization’s administrator if the PC is managed. If a third-party antivirus remains registered after normal removal, follow that publisher’s removal guidance or ask its support team. For other errors, the message and device details are needed to choose a safe next step; there is no single registry, service, or policy repair established for every case.
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