He said he tried VLC, and it didn't work. Although, if you don't have the latest version I would recommend updating to the newest release. Also, make sure you have the latest version of the 3ivx or Divx plugin, and then you could try playing the file in Quicktime.
You don't know very far. Whether or not you pay the $29 for QuickTime Pro has nothing to do with it. You need the DivX codec and/or the 3ivx codec. These play DivX-encoded .avi files. For old fashioned .avi files, you need the Indeo codecs. They, however, only work in Classic. Indeo 3, 4, and 5 have never been ported to MacOS X.