don't be STUPID.
DON'T INSTALL THE XVID CODEC (yes i'm shouting)
install FDDshow filter that will decode
Dvix
Xvid
WMV
Mpeg-1-2-4
and other formats as well.
it is VERY good , and is NOT A CODEC PACK.(by installing to many codecs you may cause conflicts that may render all movies anwatchable, that is why a single filter for as many formats is the best method)
i know about codecs and this is the best and most stable for all your needs.
get it from here.
http/www.doom9.org/Soft21/Filters/ffdshow-20021213.exe
it doesn't ENCODE so you can't create Xvid movies with it, but you can certainly watch them and also at a much lower CPU usage than the official Xvid Codec.
hope that helps
Alltaken