Most computers come with fans to keep them running cool enough to avoid problems. Unfortunately, fans have moving parts, move fast, and stir air. All this results in quite a bit of noise which can cause headaches, not to mention being down right annoying. It also causes a lot of vibration which further adds to the noise, headaches, and annoyance. Some people get rid of their fans and use alternative cooling methods, however most of these are just too expensive for the regular computer user, and worse, too complicated. Good computer cases will come with rubber feet instead of the hard plastic ones that cheaper cases have. The rubber feet will absorb the vibrations instead of passing them to the desk, floor, or whatever surface the computer rests on. If your case does not have rubber feet, then you can add them if you are comfortable/able to open the case. If not (or even if your case does have rubber feet), there is a quick, easy, and cheap trick that can help to reduce vibration which will result in less noise and provide quieter, smoother computing environment.
Simply place a couple of foam-rubber mouse-pads beneath the computer. You’ve probably got a few lying around somewhere, or can easily buy a couple for under a dollar. The design does not matter, so you can get generic ones for less. The thick, foam-rubber kind work best, although even the thin rubber kinds are better than nothing. In either case, you just slip in under the computer to cushion it from the desk, making sure that the rubber side faces the computer. Even better, fold it in half (rubber side out), and you’ve got a vibration damping, noise canceling, double-cushioned, non-slip shim between the computer and the desk.
*UPDATE*
Here is a blog entry which says pretty much the same thing and includes pictures.