computer resetting?

Two days ago my computer kept resetting itself, every time I started it up it would go to the screen where it says windows is starting up them it would reset its self, I managed to start it up in safe mode then did a system restore, but it still kept ressetting. I finally gave up and turned off the computer at the mains, this I left for a few hours. When I eventually restarted it was okay and it has been ever since, but there must of been some sort of problem some where. Any Ideas Please Help