Have you checked to see if the machine is still under manufacturer warranty?
PCs that i have worked with in the past that are showing this type of behavior have led to either the hard drive (HD) being bad or the motherboard (MoBo) beginning to fail; in some instances, it could be a failing power supply as well, but i think the machine would not even boot back up if the power supply were to blame.
If any of these items are still covered under warranty, contact the manufacturer's tech support or warranty support ASAP, let them know the issue you are having. They may want to perform a few tests over the phone with you to verify the hardware failure and pinpoint the piece of equipment that is causing the unintended system reboot.
Not sure what manufacturer makes your particular PC; but if its a Dell, they are pretty good about working with customers to troubleshoot these failures and are pretty compliant when there are known hardware issues they simply run a few tests with you over the phone and then if they can pinpoint the failure, they will send out replacement parts. It helps if you do your research and homework before talking with tech support, they tend to take you more seriously if they get the impression that you are savvy with your machine and basic troubleshooting. If you can boot to a hardware diagnostics partition or diagnostic disk to check for hardware problems, the information collected here will be gold when working with tech support and helps to speed up the process.
If its not under warranty, it still might be a good idea to contact tech support just to run through the troubleshooting steps with a professional and they can at least let you know what piece of hardware might be the culprit and you can look at replacement options based on your budget and expertise.
Reply back if you need further assistance.
Best of luck, happy hacking!
Edited by Cyb3r_Ninj@, 12 November 2007 - 12:21 PM.