Some type of malware could be causing this, but it could also be overheating.

Have you cleaned inside your computer lately? Be careful when doing this and usa a ground strap to prevent damage via static electricity. Make sure all the fans are working and not making any funny noises (case fan, CPU fan, and power supply fan).

If there's no problem there, it could be the power supply going bad. If you have another one available, try putting it in and see if it helps. If you don't have one handy, I'd try the malware stuff first.

If you suspect some type of virus, go to this thread and follow all the advice given:
http://www.daniweb.com/techtalkforums/thread5690.html

If you still suspect intruders, post a hijackthis log (explained in that thread) in the Security forum.

I had a similiar problem. My fix was the Memory. Some boards are a little touchy for some reason with certain memory brands, at least in my experience. Try re-seating EVERYTHING. Memory, Video Card, NIC, Sound, EVERYTHING. Check cables too. If your video card has a fan, make sure it's running fast enough.

If you want to get down and dirty, go into your BIOS at startup, and you should be able to find a section that allows you to see your Processor Speed, Temp and so on. See what it shows your temp as. Should be (someone correct me if I'm wrong here) around 34 Degrees? Mine hovers right around 40, but I'm over clocked too.

If you have more than one RAM module installed, and they are not of the same make and model, try removing all but one module (use it in Slot 1) and see if your system will run without hanging or restarting. Mismatched memory modules are the most likely cause.