Greg Low shows how to run SQL Server Management Studio as a different Windows user:
Now if all you want to do is to use a SQL Server login, then that’s easy. When you connect to a server in Object Explorer, or when you start a new Database Engine query, you can just choose SQL authentication instead.
But three other scenarios commonly occur.
If you need to run SSMS as an administrator on a machine with UAC, you can do this:
You right-click the link to SSMS and choose Run as administrator.
I will most commonly use option number three, when somebody gives me AD credentials for a separate server but my laptop is not a member of that domain.