Windows Vista is failing to start up.

Hello,
I've had this problem for alittle, I'm no expert on the insides of a computer and I've got a bad feeling in my stomach that it is, eg. fan problem.
But I've only seen this problem with laptops?
My computer is a desktop and regularly shuts off without any warning screen. Sometimes after this it can take me up to a dozen times attempts to switch back on, each time shutting off at the loading screen no matter what boot up option I choose (eg. safe mode, windows repair, start windows normally).
So last time it shut off I expected it would turn back on after a few turns, but I've tried and been VERY hesitant, and I tried for atleast 2 hours and seen no difference.