If you do Windows Media Video 9 or Windows Media Audio 9 encoding, you need the Windows Media Format 9.5 SDK.

Although it isn’t documented anywhere that I know of, the SDK includes these very important updates:

• The new Windows Media Video 9 Advanced Profile video codec (which implements the Advanced mode of the proposed VC-1 standard)
• Performance and quality improvements for the standard Windows Media Video 9 codec
• Performance improvements for Windows Media Audio 9

You can get them at the link below.

• Windows Media Encoder 9
• Windows Media Format 9.5 SDK