[Bullethead]

As ExMachina says, the best way to minimize the game is to run it in a window (which can be resized as desired). Do this in the display options where you set your in-game resolution--one of the options on the list is window.

If you're in full-screen mode, ALT-TAB will minimize the game. Be warned, however that most 3D games don't like to be minimized from full-screen and might not come back up correctly. This depends a lot on your hardware, what else you've got going on your system at the time that made you minimize the game, and what was going on in the game itself at the time you minimized it. So sometimes it might come back OK, sometimes it'll be hosed and you'll have to exit it.