Ok, my problem was another. Multimonitor... i've just deactivated the second one :)
What's your: resolution, videocard, videocard manager settings (if anything's changed) and DX components (via dxdiag)? Can you please provide this info, so I, perhaps, can get to the core of the problem (I know you fixed it but still)?
How to check DX components: Select the Start button and type dxdiag in the search box, then press Enter.
Go to the screen tab and tell me about the last 3 accelerations. They all should work.
I had a guy with the same problem, who had all those unavailable, and that, I suspected, might've been the cause of the problem. I believe it's all mostly in the screen resolution and how the game reacts to it.