TF2 microphone blues

Can anyone offer me some help?

I'm having a problem getting my mic to work when I'm in a game of TF2. I've used it on the mic test and it works fine and I've used the headset to communicate with others and it works fine. As far as I know, the microphone works in every way it should UNTIL I get in an actual game of TF2 and it stops working. Any idea as to why and how I could fix it?

Thanks in advance.

I can't help You since i suck at pc knowledge but i think it's important for them to know Your OS. Are You using Vista? Cause i have problems using a mic with Vista and that seems to be very common.
(Not the same problem i think but i never use a mic in TF2.)

USB or 3.5mm input?Dealge