http://www.headbands.com/gspot/download.html
might not solve your problem exactly, but many avi files especially downloaded ones can be corrupted... mostly the header info is the culprit...I use the free small avi diagnostic program "G spot" to test them and it will also check if you have a codec on your machine capable of playing the file as well as offer up info on which one to look for if it is not installed on your machine... info and download can be found at the url above...
homepage here:
http://www.headbands.com/gspot/
another great tool\info site for anyone with vid file problems is
http://www.dvdrhelp.com/tools?tool=115
hope this helps... Cya