laptop locking up

Hello,
Can anyone explain to me why a laptop would lock-up/freeze when performing complex 3d graphical calculations such as those in displaying today’s demanding games for example. It is a 3.2 ghz with HT and 1024 mb of 333 ddr ram. In addition it has a Radeon 9600 video card which should be able to handle any and all graphical calculations.
Thank you.