MPEG-1 from TVersity is a transcoded file. AVIs being encoded in DIVX is fairly normal and that will result in streaming back fine. As stated below anything that gives you an unsupported message means you need to force a transcode by accessing the #Transcode# Folder and that will create a seperate folder for each of your files. Simply enter the one which needs to be transcoded and use the various transcoding engines to play it back.
Core2Duo E8400 (@ 3.0), 8gigs of Mushkin, Vista Home Premium SP2 64bit, 2.5 TB of Space, Wired to Linksys WRT54G wired to 80GB MGS4 PS3 on Sharp 32" 720p LCD TV