Well, I have the same microphone, on the system below- I'm using the onboard Realtek codec. Using Windows 7 Pro (ordered from MS).
I've had trouble getting microphones working right; particularly, if I go into Counter-Strike, I had to run Ventrillo as Administrator, or it would not respond to the PTT key- that's while the in-game voice works fine. I know that this issue isn't the same as yours, however, while 7 has been mostly bulletproof, these issues have been popping up.
6D|| 24-105/4L IS USM|100/2.8L Macro IS USM|Canon 70-300/4-5.6 IS USM|24/2.8 IS USM|40/2.8 STM|50/1.4 USM|85/1.8 USM|
60D||15-85 F/3.5-5.6 IS USM|