If Microsoft Defender Antivirus won’t turn on in Windows 10, first check which antivirus provider is active. A compatible third-party antivirus can automatically switch Defender off; if Defender is the intended provider, turn on Real-time protection in Windows Security, install available security intelligence updates, and scan the PC. The right fix depends on the status Windows Security reports.
1. Check whether Defender is off or another antivirus is active
Open Start > Settings > Update & Security > Windows Security > Virus & threat protection. In Windows Security, select Manage providers under “Who’s protecting me?” to see which security provider is active. Labels and navigation can vary slightly on older Windows 10 releases. Microsoft’s overview of Virus & threat protection and its Windows Security scan instructions describe these controls.
- If a non-Microsoft antivirus is listed as active, Defender may be off by design rather than malfunctioning.
- If Microsoft Defender Antivirus is the provider, check its real-time protection setting next.
- If Windows Security shows no clear active provider or displays an error, note the exact message before attempting other repairs.
2. Turn on Real-time protection
In Windows Security, open Virus & threat protection > Manage settings, then switch Real-time protection on. In Windows 10, Microsoft also gives the route as Start > Settings > Update & Security > Windows Security > Virus & threat protection. See Microsoft’s Windows Security instructions.
If Windows will not let you change the setting, return to Manage providers and confirm whether another antivirus is active. A work- or school-managed PC may also have settings controlled by its administrator; do not try to override organization policy.
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3. Remove a third-party antivirus only if you no longer want it
A compatible third-party antivirus product can cause Microsoft Defender Antivirus to turn itself off automatically. If you intend to use Defender instead, uninstall the other product through Start > Settings > Apps. Follow any restart prompt, then reopen Windows Security and check the active provider and Real-time protection status. Microsoft says Defender should return to active mode after the other antivirus is removed.
Some security products do not uninstall completely or may remain registered with Windows. If the normal uninstall does not clear the product, use the publisher’s removal instructions or contact its support. Microsoft advises against running multiple antivirus tools together; see Microsoft’s guidance on protecting a PC from viruses.
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4. Update security intelligence and Windows
Security intelligence, also called definitions, helps Defender identify current threats during scans. In Windows Security, go to Virus & threat protection > Protection updates > Check for updates. Microsoft also delivers security intelligence through Windows Update. Install any pending updates and restart if prompted, then check the provider and protection status again. Microsoft explains security intelligence updates.
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, and it recommends moving to Windows 11. Checking for updates does not mean every Windows 10 device will receive new operating-system security fixes. See Microsoft’s Windows security support notice.
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5. Scan the PC after protection is restored
When protection is active—or if you suspect malware—open Virus & threat protection and run a Quick scan. Choose a full scan if you want Windows Security to check more of the PC. Microsoft’s Virus & threat protection guide describes the available scans.
If persistent malware is a concern, use Microsoft Defender Offline scan from the scan options. It restarts the PC into Windows Recovery Environment and scans without loading ordinary Windows. Save open files before starting because the computer will restart.
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6. When these steps do not fix it
There is no single established registry, service, or policy repair that applies to every remaining case. Before seeking help, record the exact Windows Security alert or error, the Windows 10 version and build, whether the PC is managed by work or school, and any antivirus products recently installed or removed.
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- For a work- or school-managed device, contact the organization’s administrator.
- If an antivirus remains registered after uninstalling it, ask its publisher for removal guidance.
- For an unexplained Defender failure on a personal PC, use the exact error and Windows build when contacting Microsoft support.
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