If you connect to the server or map a drive with the credentials you want to use first then those are the credentials that will be used for windows authentication with SQL server.
Disconnect all network connections to the server -
Open a command prompt (start-> run->enter "cmd")
type: "net use \\[servername]"
if you see any connections to the server, you must disconnect each one first with "net use /d \\[servername]\sharename"
net use \\[servername] /user[domain\[username]
you'll be prompted for your password. enter it. if you get a message about not being able to connect with different credentials, you didn't disconnect all other connections first.
Now just run SSMS as usual, and connect with windows auth. This works even if the machine you are connecting with isn't on the domain.
The same trick works with outlook/exchange. Its very handy