That's odd. Ffmpeg's xvid IS enabled:
leeh:$ ffmpeg -version
FFmpeg version SVN-r10521, Copyright (c) 2000-2007 Fabrice Bellard, et al.
configuration: --prefix=/usr --mandir=/usr/man --enable-shared --disable-debug --enable-gpl --enable-pp --enable-swscaler --enable-libdc1394 --enable-liba52 --enable-liba52bin --enable-libfaac --enable-libfaad --enable-libfaadbin --enable-libgsm --enable-libmp3lame --enable-libogg --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-x11grab --enable-pthreads
libavutil version: 49.5.0
libavcodec version: 51.44.0
libavformat version: 51.13.3
built on Sep 17 2007 15:42:16, gcc: 4.1.2
ffmpeg SVN-r10521
libavutil 3212544
libavcodec 3353600
libavformat 3345667
I used ffplay with "-stats" to play a xvid with success until I stopped it:
leeh:$ ffplay Harry.Potter.And.The.Order.Of.The.Phoenix.avi -stats
Input #0, avi, from 'Harry.Potter.And.The.Order.Of.The.Phoenix.avi':
Duration: 02:11:50.4, start: 0.000000, bitrate: 966 kb/s
Stream #0.0: Video: mpeg4, yuv420p, 512x208, 25.00 fps(r)
Stream #0.1: Audio: mp3, 44100 Hz, stereo, 192 kb/s
invalid new backstep 590
invalid new backstep 591
11.56 A-V: 0.002 aq= 80KB vq= 84KB sq= 0B
Maybe you should try to reinstall ffmpeg just in case the files got screwed up?
Edit: Just to satisfy my curiosity, could you post the complete output of:
ffmpeg -version
Please ?