Connecting as a specific user to iMac

I am curious how one specifies a username and password when connecting to a
macintosh? I need to login as a different username prior to connecting to
the iMac. I have connected, but as a guest. I know I *could* create the same
username and password combination I currently use to login to vista home
premium, but would find it easier just to specify a username/pass via a
dialog prompt. Is this possible?

Thanks,

Mel