System Has Shutdown Computer Error

Computer: System Has Shutdown ERROR

I've been getting this message quite a few times already and I don't know what's wrong with my computer. I am doing something on the computer and then all of a sudden the screen turns black and the thing shuts down. I start it up again and I have to go to system recovery (I just did it today-again) to see if it will fix the problem but today it didn't. Now I don't know if it's because I don't restart or shutdown my computer often because it usually only happens when I don't shut it down for a couple of days when I'm not using it or when I don't restart it after a couple of days or maybe MANY hours that I haven't turned it off. It goes to a blue screen and it says some stuff..I don't remember..but it says in the bottom : system has shutdown. I've got this a couple of times already, please help me and tell em what's wrong!
P.S. I use Windows Vista (HP)