I downloaded 28 Days Later in .avi format from WinMX. I usually watch all videos, other than Quicktime, on my ATI Multimedia Player. I had sound but no picture when I opened the file. Before deleting it in disgust, I tried opening it with other players. I tried the DivX 2.1 Player. It didn't work, but I was referred to the website for an upgraded, [free], player, the 2.5.1. I downloaded it, and installed it, that the movie played fine.
http://www.divx.com/divx/player/upgrade.php
http://www.divx.com/divx/player/upgrade.php