How big are the files you're trying to play? I've found that the 400mb type of DiVX and other files just will not play for me... after going through the other media converters and pulling the video size slightly down from the native 800x480 of the screen to something smaller - read this article... it saved me a few headaches.

also, ask the members here. the collection of knowledge here is incredible... and I'm still just a n00b learning.

hope that helps you a bit. and good luck.