Can't turn the security on (win 8.1 x64)

OK, first try to change 'DisableAntiSpyware' and 'DisableAntiVirus' to 0 then try 'DisableRealtimeMonitoring'.

If it still do not work then close the Registry Editor and follow this:

Step 1: Download PsTools
Download SysInternals PsTools from here:
http://download.sysinternals.com/files/PSTools.zip

Step 2: Get PsExec.exe
Open PsTools and copy PsExec to C: drive.


Step 3: Start Command Prompt
Go to Start --> Type "cmd" in search box --> Right click on 'cmd.exe' and click on 'Run as Administrator'. This will open Command Prompt as Administrator.

Step 4: Run Registry Editor
Type this code and hit enter:
C:\psexec.exe -i -s regedit.exe


Step 5: Change that key

Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Defender\Real-Time Protection and change value of 'DisableRealtimeMonitoring' to 0.