I am actually posting this to find a solution for my friend. He just installed windows 7 on his computer the other day and when he tried to play his games some of them would be letter boxed.
His windows resolution is set to 1920x1080. In guild wars, he can change the resolution to 1920x1080 and it removes the black bars, however in assassin's creed when he sets the resolution to 1920x1080 the black bars are still there.
Two days ago when he played on windows vista everything was fine, it has just been since he has installed windows 7.
I've looked around on some other forums and found other similar posts, but no answers. He has up to date drivers and I have no idea what else would cause the problem.