Microphone Picks Up System Output

I'm running Windows x64 Ultimate, and my sound is going through RealTek on board sound from an EVGA x58 motherboard.

I've just got a standard headset with a pair of headphone jacks, not USB or anything. When I use the microphone, it will pick up audio that my headset is playing. I don't mean that it hears it, I mean it actually broadcasts the outbound audio stream through the microphone. Stereomix isn't enabled, and I'm really not sure what's causing it.

I updated to the latest Realtek audio driver I could find (R2.66, 10/18/2011), and that actually solved the problem... until I rebooted. After the reboot, it's back to doing the same thing. The driver is showing the correct version, so it doesn't seem to have reverted to an old driver or something. All the settings appear to be the same in the playback and recording tabs, it just went back to picking up audio output through the mic.

Any help you folks could offer would be appreciated. This has me stumped.

Oh, and I actually also tried one of those USB sound interfaces (that I normally used for a laptop), and the problem would occur even through that despite it obviously using different drivers and hardware. So there must be a setting or something I'm missing, but I'll be damned if I can find it.

Thanks.

I've resolved the issue. I got to thinking while I was at work, Stereomix isn't enabled, but Stereomix does handle that kind of output.

So if I couldn't get it to not happen by disabling it, how about enabling it but turning the volume to zero? That did the trick. Now it's properly telling my sound card to include the audio, but with no volume. I'm sure it might be using extra system resources to do it that way or something, but it's good enough for me.

Just posting the resolution in case someone else has the same problem.