Windows Media Player may not be able to play a portion of content that you select because that content requires a codec that either could not be downloaded to your computer, or is not supported by Windows Media Player. By default, Windows Media Player downloads codecs automatically when you need them to play a file.
When you are trying to play back content, temporarily set the Internet Explorer Download unsigned ActiveX controls security setting to Enable or Prompt. This setting will let you install the codec.
1. In Internet Explorer 4.x, click Internet Options on the View menu. In Internet Explorer 5 and 6, click Internet Options on the Tools menu. (Or go to Start > Control Panel > Internet Options).
2. On the Security tab, click on the Custom Level... button.
3. Scroll down to Download unsigned ActiveX controls and click Enable.
4. After codecs were downloaded, follow again steps 1-3. On step 3 change your settings back to Download unsigned ActiveX controls, Disable.
Note: If you want to avoid step 4, select Prompt instead of Enable. In this way, you'll always be notified of new "unsigned downloads".
Signed and Unsigned codecs
The following codecs are no longer signed. These codecs are available on the Microsoft codec server.
• Sharp G.726 Audio Codec
• Voxware MetaSound
• Voxware MetaVoice
• Sipro Lab Audio Telecom CODEC (ACELP.net)
As of the time that this article was published, the following decoders were signed and were available on the codec server:
• Microsoft MPEG-4 Video Codec v1
• Microsoft MPEG-4 Video Codec v2
• Microsoft MPEG-4 Video Codec v3
• Microsoft ISO MPEG4 v1.0
• Microsoft ISO MPEG4 v1.1
• Microsoft Windows Media Video v7
• Microsoft Windows Media Video v8
• Microsoft Windows Media Video 9 (DMO)
• Microsoft Windows Media Video 9 (VCM)
• Windows Media Video 9 Advanced Profile
• Microsoft Windows Media Audio 9 Professional
• Microsoft Windows Media Audio 9 Lossless
• Microsoft Windows Media Voice 9
• Microsoft Windows Media Voice 9 (DSHOW)
• Microsoft Windows Media Video Image 9
• Microsoft Windows Media Screen 9