that happens when the versions don't agree.
most likely you forgot to patch when you reinstalled the game.
you cannot use save games from a patch and use them in an earlier version of the game. they just aren't backwards compatible.
also, there are other files you may not have copied. if you took the entire "saves" file you are okay, but if you only took your last game, and skipped your "characters" file, then you could have problems.
let us know if it's the patch or not.
V***V