Did you move the installed files from their original location?
Have you modified the game, i.e. changed some files?
Failing all that, you might want to try re-installing the game.
Be sure to check out our FAQs on what you need to do (i.e. backing up your save-games) before doing it. You actually need to save files from a few places to successfully back up everything.
Good luck :: .
EDIT: Whoops. Missed the other topic. As an Admin, I can reply to locked topics, so I didn't notice. Leaving this locked.