If you have an unused fan port of any size, from 40mm to 120mm, you can install a fan. This is easy to do. They’re mounted with just 4 screws, and can be attached by a standard Molex plug by means of a 3 to 4 pin adapter. The adapter will almost always be included with the fan. Perhaps the best thing of all about these fans is their price. They cost about $5 for an 80mm, and around $10 for a 120mm.
If you don’t have a fan port, and don’t want to go to the trouble of case modification to make one, there is still a fan you can use. It’s a turbine fan, that mounts in a PCI slot. They’re thin, only the width of a slot, and somewhat disk shaped. Despite their small size, they can pull more air out of a PC than a regular 80mm fan. They can be attached to a Molex plug, and cost about $10.
Another way to attack the problem of airflow is to remove obstructions in the case. One place to start is with the IDE cables. Often called “ribbon cables” these connect your hard drive and CD/DVD to the motherboard. They can also block air to these components, allowing heat to pool. Ribbon cables can now be replaced with rounded ATA cables, which take up less space, allow air to circulate, and are much less susceptible to damage. They cost $5 each.