the default setting for generic using xvid codec & vista media center does not support xvid.
you can play it in vista windows with vlc player, or you can struggle with the geeky hacks that allegedly make vista work with xvid , but I could not get those to work.
Or you can download the free divx bundle & use the trial version of divx converter to change your xvid video into divx. vista is happier with that.
also, request divx support in the dvdfab featues request thread, if enough of us ask for it then maybe one day we’ll get it. ( and as microsoft have no intention of supporting xvid in vista, we need it! )