I dont know if a simple Codec will work in your case. it sounds like that program is trying to expand the video causing you to need not just the codec but also the files needed to re-compress the file as well. Try and download some kind of DIVX Creation software or perhaps just go here
http://www.microsoft.com/windows/windowsmedia/download/
and download the WMP9 Codecs. I have never used that program so I cant say weather what I am suggesting is correct or not, just going by what I would try.
Have you tried either sound forgue or premiere? Or do you not know anyone with these programs. I use premiere for everything, and with the new WMP9 Compression tools, you can usually recompress the video to make it look the same as before but take less space.