Computer reboot itself out of the blue. Any idea why?

i won't mention virus coz its already been mentioned. c",) . anyway, is your friend using any particular program when the computer reboots? what i mean is, is your friend using any specific program (like Word, Photoshop, or even games). sometimes, reboot is caused by corrupted software. sometimes, it is caused by incompatible hardware. . the most common cause (aside from viruses) of system reboot is worn hardware, usually the power supply itself and the cpu fan slowing down which causes overheating. it can even be caused by mere dirt inside the CPU.

I agree with the previous responder. This usually has to do with the power supply. You might need one with higher capacity, if you've added hardware that take up more than the power supply can continue to put out. This is one of the more desirable problems, as power supplies are certainly cheaper than processors. Certain activities cause the processor to work very hard. If you have a relatively new machine with a high-rated CPU and a fan, you'll be able to hear when the computer starts working hard. If your friend plays games with a heavy demand on graphics, then apparently the processor is getting too hot. You might need a new CPU fan, and perhaps additional fans throughout the chassis if mounting points are available. If that doesn't help, you might need a new processor.

My computer has been known to do the same thing, and in fact it did it yesterday. I was just surfing the net as usual, and had a few different tabs open looking at some stuff, and then it jut automatically restarted itself. Normally when it does that, it comes up with a message of some description, but nothing came up on this one. Weird.

It sounds like what was happening to mine, It was the computer that was faulty, mine was new, I dont know how old your friends is...Or maybe a virus ??

My guess is overheat (fan is not working?), your friend will need to send his computer for a service... That would be something I would do! Not worth it if the motherboard got burned!!!

heat could be a possible problem or maybe a dying PSU