Some (most?) of the stray noise may be coming from your sound card though. The inside of a PC is a noisy environment - high speed Pentium chipsets are like radio transmitters!
You can (with care!) shield your sound card by surrounding it with kitchen foil:
1)The foil must be insulated from the card (and adjacent cards), obviously - otherwise you'll have interesting smoke patterns emerging from your PC
2)You need to ground the foil to the PC case (the metal, internal case)
The above procedure has its risks, but helps. Noise also depends on the quality of the sound card - some have better filtering than others. Shielding a poor card will not help very much, unfortunately.