What causes a computer to do this?

My computer works perfectly all the time im not trying to play a game. However, when I try to play games such as HL2, CSS etc, I get this screen after about 1 min of playing.


What is the deal here?

Have you done any temp monitoring of the GPU while you are playing a game? If you have HWMonitor open in the background it will record your min, max and current temps for CPU and GPU.