What error message do you get?
Is there anything listed in the Event Viewer?
Which game?
 

I agree with the above, and would add this: Do you see the screen flash blue for a second before it restarts, if so you need to turn off the restart on error under system properties.
 
You may want to blow out your systems CPU fan in case it is heavily dust covered, it could be causing it to overheat.
 
Does the system restart during other heavily disk intensive activities like running an antivirus or spyware scan?
 
Have you installed the latest patches for the game?
 
Does this only happen on the one game or a lot of them?
 
Have you installed the latest drivers for your video card?

I did not see the screen flash blue for a second before it restarts. It just restart in the middle of the game. I have try to close everythings before playing it but that did not help. Every game i played it turn out to be the same(the only different is the time it restart). My had also help me by downloading the latest pacth but it did not help. The only things is there a note that ask me to sent an error report to Microsoft(which i just ignore it). should i try senting it????