Problem with computer

1. Most common thing is that your fan is clogged up with dust and it doesnt or hardly spins.
   Also dust on your processor chip, video card etc. is bad news because they all heat up and you dont want anything holding in the heat.
   If you're computer shuts down, then its the processor overheating, if you randomly loose the image from your screen but the computer seems to be still on, then its your video card overheating, simple as.
   
   

Keep your PC away from the radiator, that helps it cool down.
What you want to do is open up the PC tower and just hoover all the shit right out (keep the wires, bolts etc, youll need them). See if anything obstructs your fan blades from spinning, or any cracks/broken blades.

1. Power supply, could have had a bit of use and abuse and the contact in the wire gets lost. Pretty common as well, if this is your problem you're lucky beacuse power supplys are like a quid from a flea market.
   
   

Unplug the power supply and just try wiggling the wire around to see maybe its loose somewhere, if there are any cuts or something its a good idea to replace it and never use it again in the sake of safety.

1. Virus. Just some version of Trojan which tries to shut ya down.....very unlikely though, no one makes those viruses anymore, the recent ones prefer you to stay connected to the net while they download and upload crap from your computer.
   
   
   

hope this helps dude. What PC is it anyway?