SHutting down every hour.

Hello, second post here, hopefully this one gets a reply.
Every hour my computer will just shut off, and start up again by itself.
This just started happening yesterday * The 24th*, I have already tried replacing the power supply and i added a case fan to see if that was the problem, and it did not help. I am running Windows Vista Home Premium edition.
It will happen at any time, while playing tf2, searching youtube, or even if no one is on the computer.
I hope to get this fixed by tommorow *APB early release* as i am expecting one of the longest gaming sessions i have had on this computer ;).
SO any tips and fixes are appriciated, thanks for anyone who replies.

Edited by Daford, 25 June 2010 - 08:58 PM.

What anti virus programs have you run? Have you tried it in safe mode, etc.?

I run Webroot antivirus, i will try running it in safemode, and see if it works.
I ran it in safe mode and it still shut down after an hour.

Edited by Daford, 26 June 2010 - 07:56 AM.

Does it actually shut down, or just turns off?

I've seen many viruses cause it to shut down, but normally from past experiences if its just randomly turns completely off its a hardware problem.

It just turns off. i probably should have clarified that.
as in, it does not go through the regular process of shutting down as if you went to the start and ect. But as if the cord was pulled.
It does that approx. one hour after the computer is booted up, which is making it difficult for me to diagnose this by myself.

Edited by Daford, 26 June 2010 - 09:42 AM.

I restored my computers settings to last weeks back up, and it is working fine now, thanks for the replies!