If you have a Mac, you have a lot more work to do than just downloading a codec pac. First, make sure that you have all the Indeo video codecs. If you are downloading avi's, the video may open in QT, but the audio will be deranged. There are several programs to assist you. 1) DivX Doctor 2, which, with an accompanying dec (soon to be codec) will split the mp3 audio from the video and reassemble. If audio is wma, however, there is no Mac program to deal with it if DivX Dr cannot decompress the audio. If it can, you can then extract the audio in QT Pro and convert to an mp3 in iTunes or SoundJam or whatever. Have lots of time and disk space available 2) for OS X only, AVI2MOV, which does the same thing as DivX Doctor, which can run in 9 or X, 3) DivX Tool, which can separate audio from video in some movies that choke DivX Dr. You then paste the mp3 audio into the video which you have extracted with QuickTime Pro. 4)QT Mutator, which can realign audio to video if either of the previous programs messes up the synch.

All of these -- Indeo codecs, DivX Doctor 2, AVI2MOV, DivX Tool, and QT Mutator -- are linked at versiontracker.com.

There are also some mpg's that QT simply can't deal with. I have no idea why.