Odds are its in one of the other formats like xvid or maybe even one of the older krystal codecs
Check out
http://www.doom9.org for a complete set of codecs to cover pretty well all available avi and mpg formats. Also note you may need one of the older divx codecs for some even if using the latest 5.03 (the best one to cover older divx video/audio is the 3.11 alpha btw.)
NB Do not install one of the "packs" like the nemo codec pack that claim to cover everything, they don't and nearly always end up screwing with all the proper codecs instead.
Also note Media Player and the DivX player both have issues with some xvid movies so possibly try another player like powerDVD instead for any that still play up after updating your codecs.
Hope that helps