You cannot join your computer to a domain in Microsoft Windows XP Media Center Edition 2005.
Windows XP Media Center Edition 2005 does not support a scenario where you join your computer to a domain. Windows XP Media Center Edition 2005 is designed as the foundation for a premier home entertainment system. Media Center lets you share Media Center content with other locations in the home by using Media Center Extender devices. Media Center Extender devices require concurrent connections, and these connections are available only through the Fast User Switching component. Therefore, the domain join functionality in Windows XP Media Center Edition 2005 is disabled.
In earlier versions of Media Center, a scenario where you join your computer to a domain is supported. If you upgrade to Windows XP Media Center Edition 2005 from an earlier version of Media Center that is joined to a domain, Windows XP Media Center Edition 2005 will retain the domain status and credentials. If you remove the computer from the domain to enable Fast User Switching for use with a Media Center Extender, the computer will not be able to rejoin the domain.
Note You may still be able to use Windows XP Media Center Edition 2005 to access domain resources that are shared on the network. You can do this if the shared resource does not require that your computer is a member of the domain.
Media Center Technical Support
Because Microsoft Windows XP Media Center Edition was included with your hardware device or system, the hardware manufacturer provides technical support and assistance for this software. Your manufacturer may have customized the installation with unique components such as specific device drivers and optional settings to maximize the performance of that manufacturer's hardware. If you require technical help with Windows XP Media Center Edition, contact your manufacturer directly because the manufacturer is best qualified to support the software that the manufacturer has installed on the hardware.