Hi and Welcome,
Well, random reboots are usually caused by heat, memory, driver issues or power supply. You need to investigate each to find out which one is causing the reboot. If you go to my computer right click it, choose manage, and look at the event viewer you will probably see red x's right click those and choose properties this will tell you detailed info on what is causing the crash. You can also send a message to Microsoft and look at their knowledge base for additional details and possible solutions.
Another thing to try, but be careful because if the PC is overheating then this may cause more damage then good. So before you try doing this, make sure the PC isn't crashing due to overheating.
If your running Windows XP try this:
Open Control Panel\System\Advanced tab\startup & recovery box\settings button, and take the check out of Automatically restart box. Then check the Write a system event, and the Admin alert boxes. This will force a BSOD on the next crash (thats why it restarts...its crashing) that posts the Stop Error Code.
You can research that error code on Google, make sure you copy it down, exact word-for-word.
Hope that helps