I had a similar problem with Dirt2, where I had set anti aliasing too high. The game couldn't even start itself, thus I couldn't change the settings back, I tried everything (Even running in safe mode and as administrator, hoping it would change anything). In the end, I just uninstalled the game, cleaned the registry and installed it again, after that everything ran fine. If you haven't even started playing the game, I suggest you to simply re-install the game as it might just turn out to be easier and less-time consuming than changing those preference files and stuff-like that.