You didn't say which version of Windows you had. In XP, you would open the volume control, go to Options>Properties, make sure you have Volume for Playback selected, make sure you have the appropriate jack selected for display (Line In or Microphone - they are different), then go back to the slider controls and select Mute under Line In or Microphone. As long as you keep Volume for Playback selected, all you have to do in the future is open the volume control and turn mute on or off.
Volume for Recording would adjust the volume going out over your feed. You don't want to mute that.