As @Borg 386 said, the registry key fix should do it. If the key is not there, no problem - it says:
Alternatively, you can delete the Windows Defender key
from the left pane of the image shown in step 2. Finally, reboot the system
(I suppose you could also create the key, and set the value = 0)
So, after rebooting the machine, is the problem still there?
If so, you may have an infection. If sfc /scannow reported no integrity violations, then you might try an ESET online scan.