" Does it give you a warning and a countdown?
If it does you may have Sasser on it. "
No warning or countdown ...just REBOOT !
>>No warning or countdown ...just REBOOT ! <<
I'll just add that It sounds like hardware. If it looks like you just hit the reset button, that's most likely hardware. If it says "system shutting down" and thereafter reboots, it could be a number of things.
When I've seen this in the past, it's either faulty memory or bad memory timings. Depending on your Bios, you may have various default settings. You might just go into the bios and use one that says optimal and save. If the problem persists, try a "fail safe" option if available. If none of those exist, you should have an option that says, reset to factory defaults. Try it and save. It may just be that simple. Heat can also cause it though. Check for dust bunnies in the power supply and cpu heat sink.