It's a nVidia 9600 GT with the official Windows Update drivers
(191.07). Using any movie player program, everything is fine, it only happens on streaming videos. Some of the sites even write "buffering" for half a second, then they go in.
Edit, I was not right with the VirtualBox XP part. It's stuttering there too, I was just lucky before not to have stuttering in VirtualBox. (I deleted that part from my post).