Why does my computer shut down for no apparent reason?

I have Windows 7, and my pc is running smoothly. Lately, I have noticed that while I'm playing Pro Evolution Soccer 2011(running smoothly on highest graphic detail), if I press "pause" and leave the game paused for about 1-2 minutes my computer shuts down abruptly, as though the power supply has been cut. It's pretty strange because I only experience this anomaly while the game is on pause, and NOT while playing?! What can I do?