Force Windows 7 to use specific DC

Strange question.
If you have Forrest in your organization and more than one DC, then active directory should be propagated to all DC so all DC's will have same information.
If you want force workstation to use specific DC just put this DNS IP on workstation NIC IPv4 properties

If you are logged on as a domain administrator then enter the command below to create the account and join the domain

netdom /domain:savilltech /user:savillj /password:nottelling member <computer name> /joindomain

If you are not an administrator the domain admin people will have to add you an account first using either server manager or using NETDOM.EXE

netdom /domain:savilltech /user:savillj /password:nettelling member <computer name> /add

Once the account has been add

http://windowsitpro.com/windows-server/how-can-i-join-domain-command-line