Had a similiar problem with Skyp sound which was resolved by using different audio codecs. On XP Pro, have a look at control panel - sound & audio devices - hardware - audio codec properties - right click an audio codec and change the priority. If memory serves I think I upped the priority of either messenger audio codec or microsoft PCM converter to be 1. seemed to have little impact on anything else and if you make a note you can always change it back.
Rickity