Some of the standalone players work only up to a certain bitrate and/or resolution.  Also, turn off some of the xvid enhancements including qpel, gmc, etc.  Make sure you have the latest xvid version (1.0RC4 or something).  I wouldn't worry too much about standalone players, MPEG-2 is a better way to go for those.  I have no problem using someting like bbmpeg2.  MPEG-4 is such a new standard there are many compatibility problems.  If someone is able to download from the internet, they've already got a computer so I wouldn't worrey about compatibility with those players too much.