That has been mentioned before, and me and this other guy fixed it by using the AviC program located in your Xvid Directory.

Simply open up the file in the AviC program, and change both drop down menu's to DivX. (Unless it's already on DivX, in which case switch them both to Xvid.)

Should this fail to resolve your problem, you can simply swap them back to their original codec usage, and wait for someone to suggest another solution =P

Edit: oh i forgot to say. Once you've selected DivX (or Xvid, depending), in both drop down menu's. Click apply, then close the program and try the file.