Windows

The processor could be overheating. Check the processor fan to make sure it's working properly. Even if it appears okay, **** it clean with one of those nifty cans of air. You could replace it for around ten bucks.

Also check the power supply fanto make sure it's working as well.

Actually, the power supply itself could be the problem. I've seen this kind of rebooting symptom because of a bad power supply. This could be replaced for around $30-$50.

what's in event viewer?

Check your event log for entries at the time of reboot. Problems could be memory related or as mentioned before a faulty powersupply. Check memory with e.g. a tool like : Docmemory. This can be downloaded from :

http://tinyurl.com/b2x1

I agree it's likely to be hardware, but the hardware may not be bad - it may be seated poorly. Take it apart, **** it clean, put it back together. I spent months fretting over a personal PC I had with the same symptom - random reboots.

It turned out to be reseating the processor.

Another possible cause could be STOP errors if the machine is set to automatically reboot when a STOP error occurs. If you have the latest service packs installed, you should see a message in the System event log at the time of the reboot showing that a STOP error occurred and giving you the basic info on the cause of the error. If you're not getting this message, right click on My Computer, go to Properties and change the startup/shutdown properties to NOT reboot automatically when a STOP error occurs. That way, you'll see the blue screen and can troubleshoot from there. Hope this helps!