I would suggest uninstalling, using the manual uninstall instructions which you can find in the self help sticky thread under the "1 Uninstall" hot link, if necessary.
Then do a fresh install.
If you have an important savegame you want to keep then you should copy that to another folder before wiping the game. But when you reinstall try starting a new game and see if that loads before you try the old save game.
Then do a fresh install.
If you have an important savegame you want to keep then you should copy that to another folder before wiping the game. But when you reinstall try starting a new game and see if that loads before you try the old save game.