This sounds like a grounding problem. An optical (digital) connection to the receiver should fix the problem (if your receiver supports it).
Alternatively you can buy a cheap ground loop isolator from Radio Shack (Cat No 270-054). This connects to the audio cable between the PC and receiver. Mine has RCA connectors at one end (receiver line-in), a transformer-like thing in the middle, and a stereo mini-plug at the other end (PC - line out). It may slightly degrade the sound (I haven't noticed) and so the optical solution is probably the preferred choice (with the added bonus of better D/A conversion in the receiver than most sound cards can achieve).
Another option may be to ground everything using a common source. I tried this myself as a test without success, but I probably missed something.
You can sometimes fix one ground problem, only to cause another. I have a ground loop hum on video - but that's another story...