Why does my computer crash every time I play a game?

My computer never crashes unless I play a game. I've recently reinstalled Diablo 2 and Rollercoaster Tycoon 2, two games that always worked fine years ago on the same computer. I can spend hours on my computer without a single problem, but as soon as I start to play a game, within five minutes my computer randomly shuts down. any suggestions? I recently tried defragging my drive, and blowing some of the dust out of the computer.