The almost exact same things happened with me some months ago(7/8 months ago),each time I booted the pc the login window appeared, but after a very short time(10s) a error would appear and the pc would auto-restart, over and over agian, always.
I managed to save my important data in a pen, during each 10s it stayed turned on, I managed to save a documente at the time.
In this case I don't know, but it happened to me too out of nothing, strange thing.
I hope Austin can help you