If you are on X, download VideoLan(versiontracker). It should play almost anything you throw at it, unless it has AC3 or wma audio, in which case you have to decode into an aif, then recompress the audio in 9 as an mp3 and splice back into the video with QT Pro.

Unless you give more specifics, I can't help you. Use DivX Tools to find out which codec was used. DX50 and MP43, for instance, will hardly ever play properly in 9, but VideoLan can generally handle them in X. Supposedly, DivX 5 for Mac (versiontracker) can handle DX50, but so far, I have had no luck in 9 getting these to work except for one vid that was compressed at 15 fps.

"Some avi's are not even recognized". This doesn't bother DivX Doctor. Or you can change what you need with DivX Tools so it has a proper QT icon.

"The decoder and converter don't seem to work either." If they didn't work, you would only get a white screen in QT.

Usually, if the avi is corrupt, DivX Dr. truncates the audio, which you can extract with DivX Tool's slow option. If it is garbled after that, then the file was corrupt. Period. Nothing will help it, although it might play OK on a PC.