Strange Random Crashing

[shilobad]

Have Pentium 4, 2.0Ghz, 512 RAM, Geforce 4Ti Video Card.

Lately every game I install crashes after about 10-20 minutes. Everything from Tiger Woods 2006, to a stupid point and click Agatha Christie mystery that can't be testing my computer too much. The crash is always the same:
The monitor suddenly turns into a mishmash of random colors -- almost every pixel on the screen is a different color. I then need to hold the power button to turn the computer off (reset doesn't work). Upon restarting, a bit of the anomaly continues, by the time the XP login page arrives, everything is back to normal. Is this a graphics card heating problem, system heating, power supply? Any help would be greatly appreciated.