In SSMS...
Server type: Database Engine: Server name: the IP of the computer you're connecting to.
Authentication...
you can do windows, but it's harder to set up so let's not. So... make sure your SQL server supports mixed mode authentication and use the credentials of the SQL server you're connecting to, so if you're on 7 and trying to connect to xp, put in the IP of that xp box and the credentials that let you login to that xp box. Those credentials are probably SA. If not, make sure you have mixed mode & SA enabled.
This is the very basic jist of it.