MUI для x64 нет пока!
Единственное упоминание про MUI для Windows x64 есть тут:
http://www.microsoft.com/windowsxp/64bit/upgrade/faq.mspx
Q. Where will the MUI Packs be located for download?
A. MUI Packs will be available in early May by download from the client (http://www.microsoft.com/windowsxp/64bit/default.mspx) and server (http://www.microsoft.com/windowsserver2003/64bit/x64/default.mspx) pages