Messenger Restricted to Home Network Only - can I?

Hello,

Is it possible for me to setup Windows Messenger on my home network of 2
desktop PCs and 2 laptop machines all running WinXPPro SP2 WITHOUT letting it
communicate inbound or outbound beyond our home network?

Also, the two laptops are from our offices and when working via remote login
to the office messenger will need access both to our offices and the Internet
in general for business purposes.

Am I going to have to buy some tupe of server for this? If I could avoid
buying a server by just having the two desktops connected with Messenger I
would be OK with that.

Thank you very much for any help you may have to offer!
I. M. Barnfarf