If you're desperate and the video is in one of the Indeo formats (3 thru 5) you'll be able to play it in Classic if you can track down the plugins (Intel used to have them buried in their download database, but they're owned by another company now, so I'm not sure).
Try this: Open the movie (QuickTime Player should complain about it being unable to play it--click ok, not close). Now get info on the movie, and look at the video compressor; it should say something like "Indeo 5" or "MS divx" or the like. If it says divx or 3vix, the decompressor that doppelbock mentioned should do it. If it says Indeo 5, Indeo 4, or Indeo 3.2, then those Intel plugins should work. Otherwise... try waiting for Media Player from MS.