open your case up and look on your PSU for the specs, like voltage lines, amps, etc.
what kind of computer? when did you get it? specs of the PC?
it could be oldness. or a virus. try running Zone Alarm Pro and get the trial at ZoneLabs.com
did this just start randomly one day?
The other guys are possibly right although it could be due to the CPU overheating. if it's that old it may be that the CPU fan has stopped working. You may want to open up your case and turn the computer on to see if the fan is still working. if it's not you will need to buy a new one.