How to fix Windows Defender service won’t start on Windows 10

Windows Defender is more than an anti-virus app. It provides lots of other protections as well such as ransomware protection, browser-based protection against malicious websites, and on-demand file scans. 

In order to do all this, it runs several underlying services. These services start automatically if Windows Defender is enabled. A user generally does not need to intervene.

Fix Windows Defender service won’t start

Windows Defender is an essential service that runs on Windows 10. If you want to use Windows Defender, you cannot stop this service. If the service won’t start, neither will Windows Defender. Here’s what you can do if the Windows Defender service won’t start.

1. Turn off third-party anti-virus apps

One of the main reasons Windows Defender may not run or its services may not start is if you have another anti-virus app running on your system. Normally, Windows Defender does not run when another anti-virus is running.

Disable the third-party antivirus and restart the system. If the service and Windows Defender do not work, uninstall the third-party anti-virus and restart the system again.

2. Check Windows Defender service start settings

The Windows Defender service should start automatically when you boot your system. However, if the service is set to manual start, Windows Defender won’t run automatically.

  1. Tap the Win+R keyboard shortcut to open the run box.
  2. In the run box, enter services.msc and tap the Enter key.
  3. Look for the Windows Defender firewall service and double-click it.
  4. In the Startup type dropdown, select Automatic.
  5. Click the Start button.

3. Change registry settings

Windows Defender can be disabled from the registry. If it has been disabled from the registry, the Windows Defender service will not start. You need first enable it from the registry.

  1. Tap the Win+R keyboard shortcut to open the run box.
  2. In the run box, enter regedit and tap the Enter key.
  3. Go to this key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SecurityHealthService
  4. Double-click the Start value and set it to 2.
  5. Restart the system.
  6. Enable Windows Defender.

4. Check Environment variables

Users have the freedom to edit their Environment Variables however, users shouldn’t remove any of the paths that are added to it by default. Check if the path to the %ProgramData% variable is set correctly. If it isn’t, it will prevent Windows Defender from working.

  1. Open File Explorer.
  2. Paste this explorer shell:::{BB06C0E4-D293-4f75-8A90-CB05B6477EEE} in the location bar and tap the Enter key.
  3. Click Advanced system settings in the column on the left. 
  4. Go to the Advanced tab and click Environment variables.
  5. Select %ProgramData% and it should point to C:ProgramData. If it doesn’t, edit it and update the path.
  6. Click Ok and restart the system.

5. Change permission for ProgramData

The ProgramData folder is an important system folder. If the folder’s permissions have been changed, it will impact Windows Defender. 

  1. Open File Explorer.
  2. Open the C drive or whichever is the Windows drive.
  3. Right-click the ProgramData folder and select properties from the context menu.
  4. Go to the Security tab.
  5. Click Advanced at the bottom
  6. Disable all inherited permissions.
  7. Click Apply and restart the system.

Note: ProgramData is a hidden folder. You must enable hidden items in order to access it.

6. Run a system scan

If files on Windows 10 have been damaged, Windows Defender won’t start. You can use the SFC scan tool to check for and repair damaged files.

  1. Open Command Prompt with admin rights.
  2. Run this command: scf /scannow.
  3. Allow the command to complete and enable Windows Defender.

7. Install pending updates

Windows Defender receives regular virus definition updates. If these updates aren’t installed, you may run into problems getting the anti-virus to run.

  1. Open the Settings app with the Win+I keyboard shortcut.
  2. Go to Update & Security.
  3. Select the Windows Update tab.
  4. Click Check for updates.
  5. Install available updates and restart the system.

8. Create a new user

It’s possible that the files for the current user have been damaged beyond repair. The easiest way to check this is to create a new user and enable Windows Defender. If it is enabled, you can switch to the new user, make it the system admin, and remove the old account.

  1. Open the Settings app with the Win+I keyboard shortcut.
  2. Go to Accounts.
  3. Select Family & Other people.
  4. Click Add someone else to this PC.
  5. Configure a new, local account.
  6. Sign in to the new account and enable Windows Defender. 

9. Reset Windows 10

If creating a new user doesn’t work, it’s time for the nuclear option; reset Windows 10. You will keep all your files but the apps will be removed. 

  1. Open the Settings app with the Win+I keyboard shortcut.
  2. Go to Update & Security.
  3. Select the Recovery tab.
  4. Click Get Started under Reset this PC.
  5. Choose the option that allows you to keep your files.
  6. Follow the on-screen instructions and the PC will be reset.


A computer should not be without an anti-virus. If Windows Defender isn’t running, and you have no other anti-virus app in place, your system is in danger of being infected. The above fixes should get Windows Defender up and running.