how do I reinstall windows defender for windows 10

Yes, this happened to me, too. Apparently, Win 10 thinks Spybot is anti-virus, even though I just installed the free (no anti-virus) version. Even if you uninstall Spybot, there is a program that periodically asks you to reinstall it.

 I removed all mention of Spybot that I could find from the registry, but still can't start Defender.

Hi,

To enable Windows Defender.

1. You may have to uninstall Avast to enable Windows Defender.
2. To uninstall Avast, follow the steps.

• Press Windows key + X and click on Control Panel.
• Under Programs, click on Uninstall a Program.
• Search and click on Avast. Then, select uninstall.

3. Once you have successfully uninstalled Avast, press Windows key and go to Settings > Update & Security > Windows Defender

4. Check (Switch ON) all the boxes. 

Hope this information helps. Please let us know if you need any other assistance with Windows in future. We will be happy to assist you.

Windows Defender is integrated in Windows 10 and cannot be uninstalled. However, when a third-party antivirus app is installed, it will register with the Security Center and Windows Defender will be turned off. This is by design, and is necessary to prevent conflicts that will occur when any two real-time AV apps are running concurrently. When a third-party AV app has been properly installed and you try to start Windows Defender, this is what you should see:

But if Windows Defender didn’t get properly disabled, then you’ll see an “At risk” Windows Defender UI with a message stating that the service is stopped and needs to be restarted. If you then attempt to start the Windows Defender Service in the UI, this will fail because “The group or resource is not in the correct state to perform the requested operation” (Error 0x8007139f).

If you then attempt to start the Windows Defender Service in services.msc, you’ll get Error 577. This is a false alarm, and is merely an artifact of the underlying error state. So there’s really nothing to be concerned about – there aren’t any problems with the digital signature or with malware. You might also have the associated issue where Windows Defender definition updates fail to install with Error 0x80070643. These are all symptoms resulting from a failure to disable Windows Defender in its own registry key when a third-party AV app is installed – as I’ve demonstrated experimentally in this thread:

http://answers.microsoft.com/en-us/protect/forum/protect\_defender-protect\_start/windows-defender-error-code-0x8007139f/93096739-16ce-471b-8716-1776f0c69f89?page=2

So if you want to use Windows Defender as your antivirus provider, then you’ll need to uninstall and run the removal tool for the third-party antivirus product that you’ve installed, or that came preinstalled on the PC. On the other hand; if you want to use the third-party AV app, then you’ll need to completely disable Windows Defender. You might be able to do that just by uninstalling the third-party AV app, running its removal tool, and then reinstalling with the latest version. But that will sometimes fail; and if it does fail (or if you just want a quick fix for this issue), then you should be able to completely disable Windows Defender with the Ten Forms Turn_Off_Windows_Defender.reg file:

http://www.tenforums.com/tutorials/5918-windows-defender-turn-off-windows-10-a.html

The Ten Forums reg file adds a DisableAntiSpywareDWORD to the Policies registry key and sets its value to 1. When I did this manually on the test machine, it sucessfully turned off the Windows Defender UI, and it also fixed the definition update issue – but it didn’t fix the issue with Error 577 in services.msc. So using the Ten Forums reg file to disable Windows Defender with the Group Policy switch is clearly only a workaround for the issue. Nevertheless, I was able to confirm that Error 577 is a false alarm, as indicated above. In order to fully resolve this issue, the DisableAntiSpywareand DisableAntiVirus DWORDs in the Windows Defender registry key (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Defender) need to be set to the correct value of 1, as I’ve explained in this recent thread:

http://answers.microsoft.com/en-us/protect/forum/protect\_defender-protect\_updating/windows-10-desktop-pc-kb2267602-definition/d5b551bc-bf40-46d8-a05e-924d33882d6d

GreginMich

some how MPC cleaner has gotten in my computer and blocks any attempt to delete or reinstall defender. Can you help?

Thanks - instinctevly that sounds right when I first downloaded the other anti virus program it automatically turned off defender so maybe the remnants are still there as you suggest - I'll hunt around & see how I go or if no luck subscribe & pay for a commercial program