If you have, Microsoft Security essentials installed, or any other full fledged Anti-Virus installed, Windows Defender will not turn on. If you had it running before, and you installed, an AV Program, then WD, is turned off by default.
Turn Windows Defender on or off .
http://windows.microsoft.com/en-us/windows/turn-windows-defender-on-off#turn-windows-defender-on-off=windows-7