Navigate to the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
If you see the preference DisableAntiSpyware on the right double-click on it and set it to 1 to disable Windows Defender.
If the preference does not exist, right-click on Windows Defender and select New > Dword (32-bit) Value, and name it DisableAntiSpyware.
Go to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender\Real-Time Protection
Right-click on Real-Time Protection and select New > Dword (32-bit) Value, and name it DisableRealtimeMonitoring.
Set the value to 1.
To restore Windows Defender at a later point in time, delete the key or set its value to 0.