Windows Defender isn’t supposed to be turned off via Group Policy when a third-party AV app is installed, as I’ve explained in this thread:

https://answers.microsoft.com/en-us/protect/forum/protect_defender-protect_start/windows-defender-is-off-i-cant-turn-it-on/c1ef0d9b-7940-4108-9578-de2c996b75ea

So on a home PC; we would normally assume that the Group Policy switch was thrown by malware that’s somehow managed to escalate its privilege-level to the rank of “administrator” – and we would recommend scanning with this set of malware-removal tools:

Kaspersky Virus Removal Tool:

http://support.kaspersky.com/viruses/kvrt2015

Emsisoft Emergency Kit:

http://www.emsisoft.com/en/software/eek/

Malwarebytes Anti-Malware (free version only):

https://www.malwarebytes.org/antimalware/

Eset Online Scanner:

http://www.eset.com/us/online-scanner/

Some other trusted third-party malware-removal tools are listed here:

https://answers.microsoft.com/en-us/protect/wiki/protect_other-protect_scanning/list-of-malware-removal-tools/d824b9af-ebd8-4c47-94e2-8ee6c544c100

Then turn Defender back on with this REG command: Right-click on the Start button, select Command Prompt (Admin), and then copy, paste, and run (enter) this command line:

REG DELETE "HKLM\SOFTWARE\Policies\Microsoft\Windows Defender" /v DisableAntiSpyware 

As I’ve illustrated here:

https://answers.microsoft.com/en-us/protect/forum/protect_defender-protect_scanning/windows-defender-this-app-is-turned-off-by-group/dfa59c2a-b358-4f62-80b6-6b8e4994dc32

Or, if you suffer from command-line phobia, then use the REG file provided here:

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

Turn_On_Windows_Defender.reg is a trusted file, and this is all there is to it:

Windows Registry Editor Version 5.00

; Created by: Shawn Brink

; http://www.tenforums.com

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

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender]

"DisableAntiSpyware"=-

GreginMich