PC reboot itself

It's a system problem you (your windows) have and there is a option in windows to restart the PC when it's like this. It's not easy to find out what's the problem but you can easily deactivate the restart. Just go in the system properties (right click on my computer and chose properties) and under advance properties you have a button for startup and ... Click on this and there you have (around the middle) a checkbox under system that says restart auto. Uncheck it and it will not start again.
About the problem that cause this, you should check your logs (that windows create in this case) to try to solve your problem.

This is likely a variant of the worm that went out this week. The worm can infect your computer as soon as it comes up, even if you have reinstalled windows. The worm will infect your computer even if you have not downloaded anything.

Here are possible solutions:

1. Put the machine behind a firewall of any type and then run windows update. (If you are currently behind a firewall, then the worm is somewhere on your network, shut down the other machines.)

2. Download all the patches for Windows XP pro on a different computer, copy to a CD, and then apply to the machine.

Have you checked your motherboard over?
Look at the capacitors around the CPU.
If they are still flat on top they are good.
If one or more are raised than the capacitors are bad. You will have to replace the motherboard.

About 3 to 4 years ago a manufactor capacitor had a alot of bad capacitors made, a number of motherboards have them installed.

You can replace the bad capacitors. here is a site that http://www.motherboardrepair.com/

Good Luck