Okay, got it to work. I installed 8.12 vista driver (manually, first removed catalyst drivers, then through the device manager, automatic installation won't work). Now I have more or less usable GUI, though it does flicker when in full screen and when I'm moving the mouse around. So I guess it's Catalyst after all...
zaelu, I also lost the activation, same thing here bro... ah what can you do, you pay for not paying attention...
EDIT: Installed the Win 7 preview Catalyst - and no change (but the GUI is not white anymore). And the GUI flickering gets worse if I turn off Aero! Plain weirdness. Catalysts are flaky, guess I'll be buying nVidia next...
@Lobo - thx for the link, Jesse's util works like a charm!