My PC restarts with no-reason at every 15min.

Hi,

I have a problem with my computer.
It starts at every 15-20min and if i download something or making a document i lost all work. (or download progress)
I have an ANTIVIRUS (Microsoft Security Essentials)
I maked 3 quick scans and 1 complete but no result.
I have WIN 7 ENTERPRISE.


Please help me,

-lzongolo