In DS 7.1, sometimes when sending Win 7 images, the unattend.txt file is ... broken and fails to join the domain. It also breaks the post-configuration task. We don't have full documentation yet on why this is happening, but we do have a fix!
Attached at the right is an unattend.xml file (decompress the zip) you can use as a custom unattend.txt on your imaging task (not on capture, on deploy). You may need to customize it. However, the key is that it is missing all of the domain related sections so that it wont break that part.