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I’ve come across a scenario on a couple of client sites whereby new PCs are staged in a separate VLAN (away from the main network) and fail to join the domain. It is usually a name resolution issue and is resolved by changing the domain name in the unattend.txt file from DNS format to the NetBIOS format (or vice versa).
On a related note, Microsoft knowledge base article 299969 gives advice and guidance on creating a non-administrative account to join the domain as the username and password are stored in clear text in the Windows XP unattend.txt file and cannot be encrypted.