freezing problem!

hello /computers/!
basically my computer, running windows xp, has aquired the nasty habit of freezing after about 20 minutes of use. this can be from general web browsing or music playing, to more intensive stuff like watching a video or running a game, to even just being left idle after start up.
it locks up without any error message and nothing upon rebooting.

any idea on how i could go about diagnosing what the problem is? is there a log made somewhere in the computer to show what's happened/caused the lockup? could a virius cause the system to just freeze like this?

Don't do that. Windows pitches a fit when transferred to a different computer without warning, unless the hardware is very similar. Download a trial of NOD32 and scan for viruses, then try borrowing another power supply (make sure it's rated high enough for your PC - you won't learn anything by sticking another underpowered PSU in there).