I believe that Windows Media Player does not play .avi files because I tried viewing my Ugly Betty episodes on the x51v and WMP does not support that file type. What I did was download a player that played that kind of file type and many others. Here's the link: Video player (The Core - TCPMP) - plays DIVX & other video types (install on Default/BIS/SD/CF memory):
TCPMP.
RapidShare: Easy Filehosting [or just copy and paste that into your browser's search bar] =). Hope that helps!