Click the Start W7 globe, in the Start Search box, type in: services.msc, and then press Enter.
When 'services' appears at the top of the search area, double click on it.
In the Services window, scroll down to >Security Center< and double click to open it.
Does it show service Status as: Stopped?
If not, what does it say?
In any event, select: Automatic and select: Start.
If the service fails to start, an on-screen error message appears.
Please post the error information in your reply.
Retired - Doin' Dis, Dat, and slapping malware.