Here's what Microsoft suggests:
"You can play back .mp4 media files in Windows Media Player when you install DirectShow-compatible MPEG-4 decoder packs. DirectShow-compatible MPEG-4 decoder packs include the Ligos LSX-MPEG Player and the EnvivioTV.
For more information about the Ligos LSX-MPEG Player, visit the following Ligos Web site:
http://www.ligos.com
For more information about EnvivioTV , visit the following Envivio Web site:
http://www.envivio.com/products/"