Remember there are many, many versions of Xvid and some of them really do suck. I have had massive problems with Xvid, from flipped screens to black screens to green screens to screens that look like I took too much acid(or just enough hehe). I suggest going HERE to get a GOOD version of Xvid, since this is the one that I think worked for me:
Koepi' stable build:
http://www.roeder.goe.net/~koepi/
XviD-04102002-1 _ALPHA_ Release
Based on CVS from 04.10.2002 07:30h MET
PS uninstall all other xvids and delete anything xvid including .ax and .dll from your sytem first, REBOOT then install this Xvid and pray.