Auto-volume adjust, disabling in XP?

[Pink Panther]

My problem is a little bit complicated:

I want to disable the automatic volume decrease in Source games (say, Team Fortress 2) when someone is talking in-game/using their microphone. I've dug for hours through Google articles and console commands and with the help of a friend who wanted the same thing, learned there is a fix. He is using Windows 7, and fixed his problem by using the change in the Communications tab mentioned here, here, and here. It looks like this.

My question/problem is: Is there a comparable feature in Windows XP? Or is there a hidden console command somewhere in the engine?

Any assistance is appreciated.