Windows 7 - Joining a DNS Domain through sysprep unattend

I'm trying to deploy windows 7 to a bunch of computers in our company using sysprep unattend.
I've got it working to the point where it runs through, gives itself a name and all that. The past 2 days I've been trying to add it to our domain but it's always prompting for a new user name OR password change.
If I try to enter the domain user (eg. domain\myname) then it tells me the user already exists, so it's obviously connecting to the dns. Even if I tell sysprep to use that name and password for the autologon, it still asks me for a new user.

Is there a way to get past these? It's driving me insane.