If you have a Windows Media Player (WMP) video or audio files that you can't view or listen to, this can be caused by the WMP missing the CODECs required to access the content. CODEC stands for COmpression/DECompression, basically it is a program that is used to compress video at different quality levels. Some CODECs are really good at creating high quality video/audio files, while keeping the file size small.
The WMP comes with a default set of CODECs that should be able to play most audio/video files. Although, it is not uncommon for video files to be compressed with the popular DivX CODEC. If you try to play a file encoded wtih this CODEC and you don't have this CODEC installed on your system, WMP will not be able to display it.
In order to view these files, you need to download and install the DivX software on your computer.