This is mostly a result of an error in your registry, e.g. invalid key, and can normally be resolved with clean re-installation of the game. To do so, follow this instruction:
Make sure you have the original CD key to avoid annoying problems!
You need to reinstall this game as well.
What is this a solution for?
Reinstalling BfME can solve most of the most usual problems in a robust and fast way. It can solve some of the usual game.dat and missing file problems and isn't hard to do to anyone.
Is it easy and will it take long?
This won't take long and it's easy to do for anyone.