How to demux without reencode ?

Most of the things I figured out, but I'm still looking for an option to determine the Xvid Codecs exact Version and the enhancements used. I really hate the codec not supported message which I get about every second Xvid File I try on my DVD Player. Sometimes they seems to be identical, same frame size, framerate, audio, codec, unfortunately only one of them works. Have already updated the firmware, must be some difference with the Xvid coding used.