sounds like you've got something in there. trojan, virus, whatnot.

sometimes a computer automatically shuts itself off when it overheats, as well, when the fans inside are faulty or have stopped working because they're clogged up with dust and webs. it's a protective measure to make it from overheating the motherboard. you'd notice it if suddenly, your computer is noisier than normal, or if it seems really hot, or both.

but, myself, i think it sounds like a trojan or a virus, or spyware.

let the scan run and see what happens, and update here? i'll help you out if i can or direct you to a site that'll help.