I have the HD264 Pack for Windows Media Player installed, but I only get audio when I play back some .mp4 files (i.e. no video). Why?

.mp4 files are either in the H.264 or MPEG-4 formats. The HD264 Pack for Windows Media Player is for .mp4 files in the H.264 format. To play an .mp4 file in the MPEG-4 format with Windows Media Player, you must first install DivX or another MPEG-4 codec.