Which one are you trying to watch, the Quicktime or the Xvid?
For the quicktime version, make sure you have an updated quicktime.
For the Xvid version, make sure you have the Xvid codec installed.
I am currently exploring some options to stream these videos using the flv format, or flash video file. They will stream as they play, and have a high compatability. I will still leave the .Mov and .Avi files up, as you can’t save .flv movies for later.