Uninstall third-party antivirus software and use official removal tools if needed, as leftover components can disable Windows Security. 2. Run the built-in Windows Security troubleshooter via Settings to automatically fix common issues. 3. Ensure essential Windows Security services (Security Center, Windows Defender Antivirus Service, and Network Inspection Service) are set to Automatic and currently running in services.msc. 4. Repair system file corruption by running sfc /scannow and DISM /Online /Cleanup-Image /RestoreHealth in an elevated Command Prompt, then restart. 5. Check Group Policy (gpedit.msc) or Registry settings to ensure Microsoft Defender Antivirus is not disabled, or use PowerShell commands to verify and enable real-time protection. 6. Install pending Windows updates, as outdated builds may contain bugs affecting Windows Security. Following these steps in order typically restores Windows Security functionality completely.

If Windows Security or Microsoft Defender isn't turning on, it can leave your system vulnerable. This issue is more common than you'd think, and while it can be frustrating, there are several reliable fixes you can try. The root cause is often conflicting software, corrupted system files, or misconfigured settings.

Here’s what usually works:

1. Check for Third-Party Antivirus Software

Windows Security (including Defender) automatically disables itself when it detects another antivirus program. Even if you uninstalled third-party antivirus software recently, leftover components might still be registered in the system.

What to do:

• Press Win I → Go to Apps → Installed apps
• Look for any security or antivirus apps (e.g., McAfee, Norton, Avast, etc.)
• Uninstall them completely
• Restart your computer

After rebooting, Windows Security should reactivate automatically.

? Tip: Use the official removal tools from antivirus vendors (like Norton Remove Tool or McAfee Consumer Product Removal) if standard uninstall fails.

2. Run the Windows Security Troubleshooter

Microsoft provides a built-in troubleshooter that can automatically detect and fix common issues with Windows Security.

Steps:

• Go to Settings → System → Troubleshoot → Other troubleshooters
• Find Windows Security and click Run
• Follow the prompts and apply any recommended fixes

This often resolves service-related glitches preventing Defender from starting.

3. Ensure Windows Security Services Are Running

Defender relies on background services. If they’re disabled or stuck, the app won’t turn on.

Check the services:

1. Press Win R, type services.msc, and press Enter
2. Look for these services:
   • Security Center (also called “Security Services”)
   • Windows Defender Antivirus Service (sensecrtsvc)
   • Windows Defender Antivirus Network Inspection Service
3. Double-click each one and make sure:
   • Startup type is set to Automatic
   • Service status shows Running — if not, click Start

If services won’t start, you may need to fix system corruption (see next step).

4. Scan for System File Corruption

Corrupted system files can break Windows Security.

Run these commands in an elevated Command Prompt:

1. Press Win X → Terminal (Admin) or Command Prompt (Admin)
2. Type the following commands one at a time, pressing Enter after each:

sfc /scannow

DISM /Online /Cleanup-Image /RestoreHealth

• sfc scans and repairs system files
• DISM fixes the Windows image used for repairs

Restart after both complete. Try turning on Windows Security again.

5. Check Group Policy or Registry Settings (Advanced)

In some cases, especially on work or school devices, Group Policy or registry settings may disable Defender.

For personal devices:

• Press Win R, type gpedit.msc (if you have Windows Pro/Enterprise)
• Navigate to:
  Computer Configuration → Administrative Templates → Windows Components → Microsoft Defender Antivirus
• Make sure Turn off Microsoft Defender Antivirus is set to Disabled

?? Note: Home editions don’t have gpedit. Use the Registry Editor (regedit) instead — but only if you’re comfortable. Missteps can cause system issues.

Alternatively, use PowerShell to check:

Get-MpPreference | Select DisableRealtimeMonitoring

If it returns True, real-time protection is off. You can re-enable it with:

Set-MpPreference -DisableRealtimeMonitoring $false

6. Update Windows

Outdated system builds may have bugs affecting Windows Security.

• Go to Settings → Windows Update
• Install all pending updates
• Restart if needed

Major updates often include security component fixes.

Final Thoughts

Most of the time, the issue comes down to leftover antivirus software or a simple service glitch. Start with uninstalling third-party AV tools and running the built-in troubleshooter. If that doesn’t work, move on to checking services and running SFC/DISM scans.

It’s not complex, but a few steps are easy to overlook — especially background services or lingering AV entries.

Basically, just work through the list. Defender usually comes back once the blockage is removed.

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