This trick could also be used with an egg carton (the cardboard one). This is an inexpensive way of helping to reduce fan noise to an extent.
I bought retail sound dampeners that I glued to the side panels of the PC case (in my old desktop), and the sound was reduced by alittle bit, but was a noticeable change.
If you really want to reduce the noise coming out of your PC, find a CPU fan thats good enough for your processor (like the one found along with retail, non OEM, CPUs), because the loud ones are more for overkill cooling: designed with overclockers in mind. You can find some fairly quiet solutions with elaborate heatsinks, but I havnt tried these yet.
You can also find some quiet power supplies at your local retail chain.
If you have a MICROCENTER in your area, you'd be in luck (its my favorite computer retail shop).