solution,c'est un prob courant,reponse trouvée sur un forum anglais:
I don't know if you can do the same for an ATI GPU, I have Nvidia. But what I did was go to the Nvidia control panel, add this game, and set it to use the Nvidia graphics card instead of integrated graphics card. If you have an intel core processor, it is likely that the game wanted to use the integrated graphics card instead of the actual graphics card.
I also believe that if you go to the game's folder on Windows 8 and right click on the .exe, then you will find a submenu that says "Run with graphics processor", You can choose to use your GPU instead of the integrated graphics card. However, I'm not sure if it's just my computer or if it works with anyone else's.