If you think your computer or laptop is overheating, follow these tips to solve the problem:
1) To confirm that overheating is the problem, install CPUID's Hardware Monitor (free download here):
Install Hardware Monitor and run it. The temperature of your CPU should be lower than 70 C for a laptop (lower than 60 C for a desktop PC). The temperature of your video card should be lower than 60, even if you're running a complex 3D game. The temperature of your HD depends on your manufacturer, but it should never be higher than 45 C (55 C for a 10,000 rpm driver)
2) To keep computer heat as low as possible, clean up your laptop or desktop regularly, removing dust and dirt. Remove the computer cover and use a compressed air can to blow the dirt out. Also, take care of verifying that the fan(s) move easily.
3) If you have a desktop computer, avoid placing it directly on the floor, since they will collect more dust thsi way. Place it on your desk. Also, avoid putting it inside any enclosure. Allow at least 3 inches between the sides of the case and a wall to allow plenty of air circulation.
4) If you have a laptop, avoid using it on top of any fabric (blanket, bed, pillow, etc). This blocks most of the airflow, causing overheating.
5) Laptops are not optimally designed for heavy gaming, which stresses your CPU and video card, producing very large amounts of heat. In other words, if your laptop overheats while playing 3d games, accept that this may be just normal for your machine... or play your games on a desktop PC.
6) Overclocking your CPU, GPU and/or FSB will produce much more heat than normal. To avoid the extra heat, do not overclock.
Read my other tips to keep your desktop or laptop computer running at top efficiency, including some tips about speeding up a slow computer.