N wanted to use an XP machine as a gateway for a LAN which is normally not connected to the internet. Googled it and found that to use Windows XP as a router, it's a simple registry edit,
HKEY_LOCAL_MACHINESYSTEM -> CurrentControlSet -> Services -> TcpipParameters -> IPEnableRouter
should be set to 1.
Step-by-step instructions on how to set gateway properties on each of the machines etc is given at