As a consultant, I often find myself needing to access clients’ systems from my own PC. If you need to access a SQL Server on a different domain, it should be simple to change the domain\username in the SQL Server Management Studio Connect to Database dialog. However, you will quickly discover that User name and Password are disabled when you select Windows Authentication.
To connect to a database server on a domain your machine is not bound to, simply start SSMS using the runas command with the /netonly option.
I keep a batch file on my system that will allow me to start SSMS using any domain authentication.
set /p domain="Enter Domain: " %=%
set /p username="Enter Username: " %=%
runas /netonly /user:%domain%\%username% "C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe"
When you run this batch file, you will be prompted for your domain, username, and password. SSMS 2008 will then start using that authentication. It will appear to display your normal login credentials, but you will be using the credentials you supplied to the batch file.