You cannot join your computer to a domain in Windows XP Media Center Edition 2005.
Media Center Edition 2005 does not support your computer to join a domain. This edition of Windows is designed as an exclusive home entertainment system. By using Media Center Extender devices you can share the entertaiments with your family members to various locations of your house..Media Center Extender devices require concurrent connections, and these are available only through the Fast User Switching component. Therefore, the domain join functionality in Windows XP Media Center Edition 2005 is disabled.
But if you have ealier versions of Media Center?
Windows XP Media Center Edition 2005 does not allow for you to join a domain but a Windows XP Media Center Edition 2004 PC upgraded to Media Center Edition 2005 can join domains.