o.k. so i downloaded a movie in .avi format it did not work with WMP, Winamp, Divx, VLC. i was then instructed to unregister the preview settings for audo and video using the Regsrv/ u shmedia.dll trick. That did not solve problem. Then i downloaded Gspot a great program that finally told me the problem was not a codec problem but that the file was TRUNCATED and 1 byte was missing. if anyone knows how to fix this problem so i can play this file i will give them my first born child. Thanks for your time.
Try and use DivFix. It detects errors in a DivX file and then fixes it (or at least attempts to ;) ) in order to make the file playable.
Whether this helped you or not, you can keep and raise your first born yourself. :)