Eu tive alguma sorte no passado com muitas AVIs quebradas / incompletas usando o DivFix , mas não tenho certeza se funcionar bem em qualquer coisa posterior ao Windows XP.
Uma versão mais recente é DivFix ++ :
DivXFix++ is designed to repair broken AVI file streams by rebuilding index part of file. This is very useful when trying to preview movies which has no index part, like some files are currently downloading from ed2k or bittorent networks. DivFix++ is complete rewrite of "DivFix" program due it's bugs and low performance.