Note this is a registry edit so if the following information looks foreign to you then stay away!
Open regedit and go to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend
Open the subkey corresponding to the service you wish to change (WinDefend), and modify the value of 'Start' to equal either "2" (for automatic), "3" (for manual) or "4" (for disabled)
That's it!
