I can tell you what my problem has been:
I'll download a 37 part rar file, an mpeg in this case, and Stuff-it won't even try to decode it. Once I've downloaded it in OS X, not even switching directly to 9.1 and using MacUnRAR works.
However, burning it to a CD, taking it to work and unraring it on a Windows machine works, so I know the file is valid...
d.