Windows defender won't switch back on.

Apr 11, 2018
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I installed Malwarebytes premium free trial and then once the trial expired, I uninstalled it. Now, I can't get Windows Defender to work properly. I can't switch on virus and threat protection. I click the restart button and either nothing happens or I get a pop-up that states I need to enabled protected folder access but I can't switch that off either.
 
Solution
Windows disables Defender, if a 3rd party solution is found on the computer and prevents re-activation until all other solutions got removed. You should check if you really got rid of all 3rd party AV tools. It might even help to install a 3rd party AV solution and immediately remove it after a reboot to manually trigger the internal check.
Apr 11, 2018
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Thanks Colif, but that doesn't really help. I don't want to use third party free software that does everything windows defender can do because that just uses up space on my computer. And I tried apremium trial that doesn't really do anything that windows defender doesn't already do too.

I also tried the regedit solution of changing the values, but it won't let me edit anything.
 
May 17, 2018
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Windows disables Defender, if a 3rd party solution is found on the computer and prevents re-activation until all other solutions got removed. You should check if you really got rid of all 3rd party AV tools. It might even help to install a 3rd party AV solution and immediately remove it after a reboot to manually trigger the internal check.
 
Solution