Could be a few things. First try running the game as administrator. If that doesn't work, try deleting the "masquerade" config file in the directory, then try again. If that doesn't work, try replacing the binkw32.dll file with one from another game, I know Oblivion's works well. Lastly, if those didn't work, try re-downloading the installer and reinstall the game, it could have been corrupt. I'm writing all this with the assumption you have no mods installed.