I am also having the same problem. Today I bought the headphone/mic combo that plugs into the soundboard and not through USB. Initially, one of the ears of the headphone worked but after a few minutes it stopped. To get the mic to work in XP try the following:
1) Open Control Panel
2) Select Sound and Audio devices
3) Select Volum tab
4) Select the 'Advanced...' button under the device volume section
5) Select the 'Options' button in the window that pops up
6) Select the Properities option
7) For the 'Playback' option find the 'Microphone' option in the scroll down list. Place a check mark beside the 'Microphone' option.
8) Close the Properities window by selecting 'OK'.
9) In the 'Play Control' Window un-mute the microphone by making sure the 'Mute' option under the microphone region has no green checkmark.
10) Cross your fingers and hope the microphone works.
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