MS-MPEG4v2 Codec is a codec based almost exclusively on FFMpeg and DivOSX. You can find them at http://ffmpeg.sf.net/ and http://mpeg4.jamby.net/, respectively. This codec plays (flawlessly, as far as I know) video encoded in the MS-MPEG4v2 codec. To install, put it in /Library/Quicktime. It will not function on Mac OS 9 or earlier. If you are trying to play an AVI file with MP3 audio, and you probably are, you must go to http://doctor.3ivx.com/ and get DivX Doctor; Quicktime has a lot of trouble playing MP3 audio in AVI files. Just use DivX Doctor on your MS-MPEG4v2 AVIs as you would DivX AVIs.
Mac OS X 10.0 or later, QuickTime 6.0 or later