Microsoft released a 64bit version of its Windows Media Encoder 9 Series.
Windows Media Encoder 9 Series x64 Edition is a tool for content producers who want to take advantage of the many encoding technologies in Windows Media 9 Series including high-quality multichannel sound and high-definition video quality on x64 platforms.
The file can be downloaded here
free of charge, and its size 12.7 MB.
The software runs on Windows Server 2003, Datacenter x64 Edition; Windows Server 2003, Enterprise x64 Edition; Windows Server 2003, Standard x64 Edition; Windows XP 64-bit. Microsoft reccomends using a 1800 MHz processor or higher, such as an Advance Micro Devices (AMD) Athlon 64 and 512 MB of RAM or more.