Windows defender isn't in services on windows 7?

when i try to open it manually from search it says "a program caused this service to stop. to start this service click the start now button or restart your computer." it closes straight after i open it. i have restarted it a lot but it doesn't work. when i click start now it says "the specified service does not exist as an installed service. ( error code 0x80070424)" i have had this problem since march so there is no point in a system restore. please don't say refresh install or windows defender is rubbish etc. if you can help me with something else can you please tell me how to fix it so that when i press the caps lock button it isn't the same as clicking off the page i am on. it is the same for word, powerpoint, internet etc. thx