What would make a computer reboot.?

A couple of years ago my pc picked up a virus that caused that. It's what is referred to in programming circles as a 'loop' where your computer keeps going around in circle and can't get past a specific point. Unfortunately I hadn't updated my CA antivirus. I had updated my antivirus definitions but not the actual program version and the version I was running didn't recognise that particular virus so couldn't remove it. I ended up having to reformat my whole hard drive. Thankfully I had most of my crucial personal files on a partition drive so I was able to reformat without losing them.