Windows Messenger 4.7 With Exchange…..

Im trying to come up with a solution for an interoffice chat system. Currently we have Exchange 2003 running and was wondering if anyone knows if it is possible to use Windows Messenger 4.7 for just local area network chatting. I was fooling around with WM 4.7 and it seems that it is forcing me to create a .NET passport account, which i do not want to do. I want to keep the chatting available only to employees on my local area network. My girlfriends company has done this with exchange 2003, but she is using version 4.6 and it seems as if her settings within that version allow her to connect to her exchange account. 4.7 on the other hand seems to be lacking this option. Please let me know. Thanks, Bob.