I never actually finished TFTD myself.
I know of a rumor which said that the comercial release of TFTD was infected by a virus known to format the HD of the player when they reached the last mission.
I am not sure if that rumor is true (or was true), but if you can't play the last mission try to find a patch for TFTD. This is likely to be enough to let you end the game.
