When I'm not on line and in the middle of playing a game using it's CD, the pc just shuts down. I have to turn it on and then it re-boots. This has happen several times. What could be the problem?

The computer actually powers off?

Do you have power options either in the BIOS or in XP set to shut down after X amount of inactivity? If you are not using the keyboard or mouse you will be considered inactive.

A gamepad, for example, does not register activity.

I got it fixed. I had my son open the case. He found a loose connection. Thanks for your help.