Somewhat related to the last post, why are computers hot and what parts are creating the heat?
Well, this one is really simple.
Pretty much, there are five sources of heat in your computer, listed below from hottest to coolest in typical computers.
1) Processor. The thingy that makes things run. This is also including the processor on your video card.
2) Hard Drive. The thingy that lets you store stuff. You might have more than one.
3) Power Supply. The thingy that converts the Alternating Current (AC) from the plug in the wall to Direct Current (DC) that your computer uses, and provides power to it.
4) Northbridge. Kinda hard to explain, it is basically a chip on the motherboard that controls extra functions of the motherboard.
5) RAM. The thingies that your computer or video card uses to temporarily store stuff.
Basically, these are the things on your computer used the most often. CD drives don't really produce all that much heat because they aren't constantly being used unlike these things.
The reason why they produce heat has to do with wasting power. The more power something takes, the more it wastes, in general. Wasted power is wasted in the form of heat, so components warm up. Sure, it is a really simple explaination, but it works.
Typically you have a fan on the Processor and Power Supply. New high end motherboard have fans on the Northbridge as well. The powersupply fan (or fans, higher end powersupplies have 2, with an extra intake fan on the inside) is an exterior fan blowing air out of the powersupply. Case fans can be purchased in order to have more air coming in or going out of your computer as well.
However, normally the only intake comes from small holes towards the front of your case, typically on the bottom. Cooler air comes in through those holes and comes out warm towards the back - so if you happen to have your computer in an airconditioned room, it might help to put the computer facing the air conditioning vent in a colder air path. Don't make the computer take intake directly in front of a heater or other heat producing devices, and make sure the computer has plenty of space for the exhaust fans to dissipate heat.
There ya go!