Problem loading the game with Windows 7 / 8 on iMac - Bootcamp

Hi all !

I have an iMac 27" mid 2011, 3,4 Ghz core i7 qith 12 Gb of 1333mhz DDR3.
My graphic card is an AMD Radeon HD 6970M 2 Gb.
The OS is Mac OS X Lion 10.7.5 (But I shouldn't matter).

I have Windows 7 and Windows 8 on a bootcamp partition, fully updated.
I tried with Apple Bootcamp drivers and also the Catalyst drivers for the GC. None of them works.

The problem is the following: I launch the game, I have the first screen (play / configuration, etc...) If I select "play", I have a black screen, have the mouse and the loading icon on the lower right corner. But it hangs and I can't go further. Even with lowest graphic options, nothing works.

Any help to resolve this issue is welcome :)