Call of Duty MW2 runs very slow on Win7

[harelme2]

I used to have CoD MW2 installed on my PC with XP SP3 and the game played beautifully. recently I've upgraded to Win7 and just installed the game on it. It runs very very slow. the first 30 seconds are kind of ok, but then it starts freezing and sometimes even crashes.
I updated my video card driver and directX -- no change.
I've also tried running with XP SP3 compatibility, and with steam in offline mode, but nothing helps.
I suspect the game doesn't use my video card. possible? how can I check?
Anyone has another idea?

Spec:
OS: Win7
CPU: QX9650
RAM: 4GB
video card: NVIDIA 9800 GT 512

Thanks