I've had a few problems with some .AVI files and after a little research, was able to resurrect them into a playable condition.

Digital Video Repair is a free small utility for repairing (fixing) AVI files. You can repair partly damaged or corrupted AVI file in these cases:
o AVI file have not completely copied from CD, DVD or other source
o AVI file have not completely downloaded from the Internet (eDonkey, BitTorrent, etc)
o Video freezed when playing
o Video can't be seeked (fast forwarded)
o Video can't be played

VLC media player - Open Source Multimedia Framework and Player is reputedly quite "forgiving" when it comes to playing imperfect or repaired media files. This page gives a rundown on using Digital Video Repair and VLC media player to get a result:
Repair Incomplete or Corrupted AVI Clips With Digital Video Repair - A Tutorial

Another avenue, which succeeded where the above didn't, was to use DivFix++ Freeware to fix incomplete AVI files. Even then, some "fixed" .AVI files still wouldn't budge. BUT, when I converted them to .mpg (MPEG-1 video/ MP2 audio) using the wonderful free program Format Factory, voila! Almost as good as new... except for missing bits of course.

Good luck
Alan