Isn't that just quaint, no mention of what went wrong at all.
Try this fix please.
http://www.twcenter.net/forums/showthread.php?t=274684
Fix didnīt work for me. The problem was that the uninstaller didnīt remove the previous BC version fully from my computer, because a part of it is located in a hidden folder under vista [User/Appdata/Local/Virtualstore/Program Files/Sega/ Medieval II..]. Once u delete the old BC folder from here before you install the 2.0 version, it should be fine.
Well, a clean M2TW version implies that you remove the old BC installation before installing the new one. The BC 2.0 installer won't touch the previous installation, so it is required to uninstall that one manually.