Headset audio

I have an Imac 20 aluminum with windows vista business installed on bootcamp. The problem I have, is the internal mic overrides the headset mic when on the windows side. I have tried to switch the preferences in the sound options in mac to headset...nothing. I then tried to configure with the altek real audio on windows side...still nothing. I only have the mac internal mic running all the time. I have had the logic board replaced and still the same thing. Isn't the headset line i/o supposed to disengage the internal mic when plugged in? If so, then this isn't happening. I am considering going to a USB headset to fix this entirely but I can't help but think that something is either wrong with the hardware or I'm doing something wrong. My mac is still under warranty and should be covered if there is a hardware problem.