I too had the same problem. My research indicated that there is a bug in the functionality of sysprep.exe in Vista, 2008, 7, 2008R2 that does not allow it to join domains. A third party developed a tool, mysysprep.exe that provides an alternative solution that allows the customization of the .xml file and will join a computer to a specified domain. See: http://tsaysoft.com/mysysprep2/