Everyone knows you're supposed to clean the dust out of your computer and keep it from overheating to keep it healthy; no one I know goes to quite the lengths that my good friend and PC maven, Ira Gershenhorn, does to make sure his PCs are dust and heat free. He recommends some pretty low-tech, inexpensive, and rather innovative things that you could be doing to avoid heat and dust problems.

If you use a desktop system, elevate the system tower as much as possible to keep it out of the dustier lower elevations in the room. Most people keep their system units on the floor. The worst place–it's guaranteed to suck up dust. If you had a choice of breathing off the floor or the middle of the room, which air would you choose?
If you have a pet, think about using a cat fur-catching rug (Ira purposefully uses a Bokhara  because he finds the weave is like one of those commercial fur-removal brushes for clothing). Fur comes off of it in globs with a damp sponge mop when you stroke it in the appropriate direction. Easier than cleaning your computer and works for dogs, too.
Next he refits a standard 20" cooling fan with a homemade cat hair filter as shown in the photo. He says he can catch a lot of fur which would otherwise end up in his PC's heatsinks.
Since the hard drive is one of the components that suffers the most extreme heat swings, he also installs a Thermaltake Smart Case Fan Series A1357 80mm Smart Case Fan II inside the case in front of the hard drives. This blows air directly on them and is regulated by the fan's thermal sensor. (He admits that it's not pretty, even though it works.) And understand that many of the new external USB hard drives are sold without fans and they can get too hot. You can buy fan cases for them, too.
Finally, if you suspect things are running too hot, most motherboards have a temperature gauge built in. Read your instructions and find out how to get into your gauge. Or you can try shareware that does the same thing; we haven't tried these.
Despite his best efforts, Ira lives in NYC which is an urban dustbowl. Eventually the dust gets in, so at any sign of trouble, you want to open the case and gently vacuum.