re:
You will more than likely find that you will have 2 adapters, one for your new headset/mic and one for the internal speakers of your pc and internal mic (if you have one).
I'd recommend opening up your sound/audio device manager, and make sure that the new adapter is selected under both Playback, and Recording sub-menu's. Also ensure that the microphone gain isn't turned up to full, mic quality will be poor if the gain is turned right up through an external headset (quality of the mic's in these tend to be a lot better than inbuilt mic's). Make sure that under playback, the speakers aren't muted and that the microphone is, and ensure that in the recording menu that the speakers are muted but the microphone is un-muted.
If you installed a 3rd party driver for the head set and it installed some management software, also double check those settings after making the ammendments to the windows audio application. In most cases the 3rd party software will naturally take precedence, but not always.
Does anyone know how to fix this? Thanks.