why computer reboots?

Problem with the computer!
My friend has problem with his computer rebooting 3-4 times a day, doesn't matter what program he uses MS office,e-mail,auto cad etc. the computer just restarts.

I have checked the cpu temp was ok, checked the ram with memtest running all night was ok, swaped power supply still crashed, updated all the drivers and MS updates still crashing.

can anyone suggest what it could be or what i could try!
thx

rocco... don't believe you're supposed to post the same question in two different sections. Although this question probably does belong here and not in MS OSes.

Regardless, it sounds like it's time for an OS reinstall.