mine use to do that all the time
found out it was something to do with my gfx card
it would run fine for about a half hour crash re boot then take its time to re boot back in to windows ... put a new card in and it goes fine now the card i had was a g force 4 mx 440 ... that card was dead from day one tho the fan on it use to make funny sounds when the computer would boot up lol
and i now have g force n card not the best but it does what i want
and it doesnt re start my computer every chance it gets lol