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Step 2: Casing the joint
If you're serious about making your PC quieter you should first ensure the case absorbs as much internal noise as possible. Many manufacturers sell cases with preinstalled sound-deadening materials, one of the best examples of which is the Silentmaxx ST11-PRO acoustically lined case. As you can see in picture 1, there's sound-deadening foam already installed in the side panels, helping to hush any noise the PC might produce. The case also has ample ventilation, so you can more easily add fanless components without risk of overheating.

Not got enough cash for a new case? Then you can modify your existing chassis. Among the best sound-deadening materials is the Acousti AcoustiPack Deluxe (v2) SE acoustic material kit. It's essentially a pack of sticky-back foam used to line the panels of your case. It comes in pre-cut packs for a small number of cases, but you can easily cut custom shapes with a pair of scissors, as you can see in picture 5. The deluxe pack comes with two foam blocks that you can shove into your 5.25-inch optical drive bays to reduce noise further.

While you're tinkering with your case, you may also consider attaching some anti-vibration feet. Vibration from inside the computer can be transmitted through the base of the chassis, causing unwanted noise--particularly if your PC sits on a hard surface. If the feet of your case are firmer than the texture of a well-done steak, simply detach them and attach some vibration-absorbing feet such as the Nexus Anti-Vibration Mounting Kit.