Computer reboots on its own

i am using windows xp and for some reason my computer will just reboot. i can be in the middle of somthing or the computer can be idle, it doesn't seem to matter. it does this randomly. i was wondering if anyone could steer me to a possible cause.

 

Open System Properties (right click MyComputer then click Properties) > Advanced tab > Startup and Recovery Settings button. Uncheck the box for "Automatically Restart" under System Failure.

This should (hopefully) cause your system to stop at a blue screen error instead of just rebooting. Note the error name, stop code, and system file name if shown. Post the information here. Example:
Stop: 0x0000000A (parameter1, parameter2, parameter3, parameter4) IRQL_NOT_LESS_OR_EQUAL*** Address x has base at x - filename

 

It would also be helpful to know what make and model of PC we're talking about here as well as it's age. Also anything you may have added such as graphics cards, memory, etc. Have you conducted virus and spyware sweeps?

 

That is usually either heat or psu caused. Have you "cleaned" inside recently?
You can take side off and blow in a little fan to see if that stops.
Also download and install Speedfan to monitor fan speeds and temps and let us know.http://www.almico.com/speedfan.php