You need to mute the microphone in the audio settings on the
soundcard on your PC.
To do this under XP, right click on the sound icon in the
task bar - select adjust audio properties - select the third tab
along 'Audio'.
Select Volume under Sound Playback and tick the mute box
under microphone. If the microphone is not visible, select the
Volume button again and choose options - properties and select the
microphone, this will then bring in the option to mute this device.
Ta
Kevan