log onto another domain?

I work for a private MD practice which has its own server and its own domain. I am a member of that domain in Win XP. We are affiliated with a hospital, lease space from them, and get our internet access through their server, which has its own domain (our server is listed in Net Neigh. under theirs).I have a user name and password for the hospital’s domain in order to access e-mail. My question is, I log on to my pc using Client for Microsoft Networks, and this only shows the practice’s domain and does not allow it to be changed. I tried using the Novell Client, I also tried setting up another user account, but no luck. I had a Win 98 machine previously and could change domains at logon at will! Any suggestions?