Water cooling is only necessary when you're doing some overvolting and overclocking.
@Fullmetal: According to HWmonitor, what is "fine"? You may want to go on the Hitman forums and see if anyone else is having issues.
[Edited by LiveFyre, 11/20/2012 6:20:21 PM]
It isn't just Hitman though. It has happened to me when I play Dishonored, X-COM, and the Assassins Creed games.
It would help a ton if you had the actual STOP error code that it gives when it restarts. You can probably find it in the Reliability History of Action Center or in the Event Viewer.
Found it. I am not seeing a stop error code though. It just says it wasn't properly shut down.
[Edited by fullmetal5550, 11/20/2012 6:59:37 PM]
I am checking the even logs though and it says something about Kernel Power errors.
[Edited by fullmetal5550, 11/20/2012 7:02:26 PM]