Thanks so much for this! It also works under Windows 7:
Control Panel / Sound / Playback Tab.
Select your headset in the list of playback devices and click Properties.
Select the Levels tab.
Click the mute/unmute icon to the right of the “Headset Microphone” level – make sure it does not show a mute indication when you are finished. You can adjust the volume of the microphone here too.
Thanks to MHough dot com and Jamie for the info. It was driving me a little crazy, too.