Hi,
I can start and play the game on Windows 7 Professional 64 bit with no problem. I notice some texture missing problem though, but that's most likely the driver's issue not the os. You should be able to play the game. My computer is an iMac, the graphic card is ATI mobility HD 4850. Usually mobile card isn't officially supported, but it's just because most companies don't even bother testing their games on one of those cards. Anyways, what I would recommend you to do is go to Microsoft's website to download
DirectX runtimes, then go to Nvidia's website to download the
newest driver.
Let me know if that helps.
EDIT: Alright, turns out my missing texture problem is easily fixed by verifying integrity of game cache. Looks like it's because some files that download from Steam server are corrupted.