I searched the forum and am still a bit confused... I use VLC and/or DivX player for most viddys. .mp4 extention files not working? Any advice?
You probably need to install the Specific MP4 codec that was used to create the File before you can watch it...
The newer versions of Quicktime can play MP4. So can Nero Showtime. If you're looking for a Directshow filter, 3ivx is a great .mp4 decoder.