Control Panel -> Sounds and Audio Devices -> Click 'Advanced' in the 'Device Volume' box.
Now, check the 'mute' box under 'Microphone'.
If there is no mute box, go to Options -> Properties, then make sure the 'Playback' bubble is clicked, and then put a check in the 'Microphone' box in the list below. Hit ok, and then put a check in the 'mute' box under 'Microphone'.
That should solve your problem.
Intel Q9400 | Gigabyte EP45-UD3P | G.Skill 2x2GB DDR2 1000 | XFX HD5870
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