Vista - Microphone channels picking up system sounds, odd playback

I'm at my wit's end with this one.

Somehow, any sound output from Vista is picked up by the two microphone devices in the Recording Devices window. No, this is not Stereo Mix- the two channels for front mic and back mic are getting the sound. The microphone slots still work, i.e., if I have no sounds playing and I speak, I can record it into Audacity. However, if a single sound plays, my speech is overridden for the duration of the sound. The output-to-microphone problem happens even when there is not a microphone in either slot.

Here's the kicker. When I turn up the volume for the microphone playback, the front mic gives me an extremely loud buzz (think "vacuum cleaner" with less whine), and the back mic gives me nothing but static and noise.

eVGA nForce 680i (122-CK-NF68-T1) - Realtek HD audio, with and without drivers produces the same effects