I've been having this problem for a while. Every time I play a game (Half-Life 2 ect.) my computer just shuts down for no particular reason (it's a laptop, btw). I can play for a couple of minutes, then my laptop starts making noices like a vacum cleaner, and then the game starts LAGGING an awful lot, then the sound usually DISAPPEARS for a couple of seconds, then the whole thing just shuts down. It hasn't been like this forever, only for a year now, and I can't remember anything from the past that made my laptop this way. What is the source of this problem, and how do I fix it? Sounds like you have a very dirty fan or one that is going bad. Start by using a vacuum around the air intake and exhaust ports on the laptop. If you are more adventuresome, get a manual and partially disassemble the laptop to get all of the dust and debris out of it. Also look to see if you can replace the fan.

If the vacuuming doesn't solve the problem and you are uncomfortable opening up the laptop - don't. Take it to a reliable repair shop.

Good luck.

EDIT: If you go inside the laptop case, don't use the vacuum, but, rather, use a can of COMPRESSED air to blow AWAY any dust.You may also install SpeedFan: http://www.almico.com/sfdownload.php, and post your laptop temperatures, while idle, and while PLAYING game.
Processor info needed.