Sounds like you might be clipping the signal,try a reduced sound level,you might have to experiment abit. turn off mic boost,test in AH,reduce record level in windows ,test in AH then try reducing playback ingame and of course test again.
When in windows check to see if mic is set for playback,if so either mute or turn levels to zero,Vista has reset the playback on my mic for some reason and I have to be sure the level was at zero as muting the mic did no good,every time I logged on it would unmute.
hope that helps.