That sounds to me as if these files are either corrupted or fakes.
I'd suggest getting a little program called "AviPreview" (google for it) that can play unfinished avi files.
That way you can check them out once you've got a part of it and make sure they are what the title says.
With the patch from winmxgroup you shouldn't get any fakes and even if one slipped through definitely not several of them.
Oh, and if I remember correctly there's a tool out there somewhere that's called "divfix" or something which can repair damaged avi files, i.e. it can generate a new index for you so you might be able to play the files you got 99% of after all.
Maybe you're lucky and they just got damaged in some way and you can at least watch them...
Good luck and please let us know if you think they're actually fakes, i.e. files that deliberately fail to download, cause that failing on the last % sounds like behaviour we know from faked songs.