How can I get stereo mix running while I have my microphone running at the same time?

I have a condenser mic (Sterling ST51) that's plugged into an external soundcard (ART USB DualPre) via XLR cable, with the soundcard itself plugged in via USB. Now, I do shows on BlogTV, and want to have stereo mix running while I have my mic running as well.. Like, I want my Winamp/iTunes/WMP/Grooveshark to be playing in the background without having the mic catch the sound from the speakers, and being able to talk to the audience as well... I'm running Windows Vista Home Premium SP2 on a Toshiba Satellite A305-S6905... I've tried enabling stereo mix and my mic at the same time, but that doesn't work, it's either one or the other, and I even tried adgusting the EnableInputMonitor registry file, though it couldn't be found for some reason. PLEASE. ANY help at all would be greatly appreciated...