OK, it's a codec you need. If the previous post didn't help, either google for "codec" and try one of the packs - KLite or DivX. I assume this is an AVI file, in which case it can contain DivX (in loads of flavours) or XVid - there is no way of telling without something like VideoInspector (
http://www.kcsoftwares.com/index.php?vtb )
Or try viewing it in something else:
dead link
If this doesn't help, let us know. Good luck!