some xvid's with the full encoding options wont run (like qpel enabled etc). As long as the xvid is encoded at A5 it will run fine in XBMP.
I just encoded tombraider2 a few days ago with the very latest Xvid 1.0 RC3 using AutoGK (see www.doom9.org to get these) - it runs fine in XBMP.
btw it was a 2gb encode with Ac3 surround and is INDISTINGUISHABLE from the real DVD. Xvid rocks !