Playback help:

Files with .mp4 extension are H.264 videos, encoded with x264.

Files with .avi are videos encoded with XviD.

Go to x264.nl and download ffdshow located under decoder. This is a great decoder that can handle both Xvid and x264 files from my site.

"During installation: Decode the following video formats with ffdshow: H.264 enables x264 playback"

If you are still having problems, for example only sound and no image, try a different player. Also on x264.nl is a link to Media Player Classic which I use, a small, versatile and free media player which requires no installation.