Join To Domain is a free to use application which can let your computer join a domain automatically, you just need to specify all the details in the application itself and then there is no user after wards to join the computer to the domain you specify.
The main purpose of this application is to input all the details required for a computer to join a domain and then see the computer joining the domain, watch the application working for you.
You can easily specify all the details including the user profile you want to backup when you want the computer to join the domain (as shown in the image below)
In the next dialog box you will need to specify the domain to join and domain admin username and password (as shown in the image below)
I have tested this application just now on windows vista and xp till now, but I am sure it will run smoothly on windows 7 also.