Q: How do I find a codec?

Answer. When trying to find the right codec for a particular file, you can use WMPlugins.com or other Web sites. Typically, you should start with WMPlugins.com first. Note that if you aren’t able to find a codec on WMPlugins.com, it is possible that the codec you are looking for is not compatible with the Player, so you might not be able to use the Player for this particular file.

We recommend that you use caution when installing codecs that aren’t listed on WMPlugins.com or Microsoft.com, such as the DivXNetworks Nimo or Tsunami codec packs. Incompatibilities are known to exist with some of the components in these codec packs that can cause serious playback issues in the Player and other players, lead to system corruption, and make it difficult for Microsoft Support to diagnose and troubleshoot playback issues. For these reasons, we strongly discourage you from installing these codec packs, and recommend that you remove them if you have installed them and are having problems with the Player. We recommend that you only install codecs, filters, or plug-ins from trusted, authorized sources, such as the Web site of the official supplier. Even then, we recommend that you use caution, because many codec suppliers offer minimal customer support. If your computer is running Microsoft Windows Millennium Edition or Windows XP, we recommend that you set a system restore point before installing any digital media components. This enables you to return to your original system configuration, if necessary.

This answer applies to Windows Media Player 10 & 11 and may also apply to previous versions of Windows Media Player.

Source: Microsoft.com