Com o Win7, isso deve ser feito automaticamente após a primeira reinicialização ...
Tente isso em este tópico:
...you can set the default logon domain in group policy so that it defaults to the domain you want. You can still find out the local station name by clicking on the link that tells you about logging on to different domains.
It is in: Computer config > Admin templates > System > Logon : Assign a default domain for logon
With this little tweak and: Computer config > Windows settings > Security Settings > Security options: Interactive Logon: Don't display last username
It makes the Vista/Win7 logon perfectly usable in a domain with less hassels than XP and staff attempting to log on locally. I actually like it better but do wish that you could customize it.