Click the "Start" button and select the "Control Panel" folder. Select the "Sound and Audio device" option from the list.
Select the "Voice" tab and choose the "Voice Playback" option from the list. If you do not see the folder, check to make sure the folder isn't hidden.
Click the "Volume" field and select the "Mute Mic" option located at the bottom. Click "Apply" or "Ok" located at the bottom of the window.
Test the setting out by opening the "Sound Recorder" from the "Start" menu. Plug the microphone into the sound card's "Microphone" slot. Press the "Record" button on the sound recorder interface.
Speak into the microphone and listen to see if you can hear your voice as you talk. Disconnect the microphone and follow the above steps again if you can still hear your voice.