Mark,
I am not a Microsoft Windows Expert but AFAIK
Java RMI relies only on
basic and plain TCP/IP Network Connectivity so you don't need to mess around with Workgroups, etc.
What you need to do and make sure is to have the IP addresses of BOTH systems running Windows in
C:\WINDOWS\system32\drivers\etc\hosts.
You can edit this text file with Notepad.
You can open a Command Prompt to find out the IP address of the system by using the command
ipconfig /all and then testing basic TCP/IP Connectivity with
ping using as the argument the IP address of the other system.
Keep in mind if any of the systems is using DHCP the IP addresses they are using might change the next time they are rebooted.
Hope this helps,
Carlos.