Hi Matt
Thanks for the response :) and the link, which will make an interesting read when I get back to the office on Monday.
The server is set up as a domain controller and two test Win7 workstations can log in to the domain, although they can't see the internet connection the server has. An IP configuration problem ... ?
The router is a Belkin and seems highly configurable. It can handle port forwarding, can be a DHCP server and has a built in firewall and a host of other features and configuration options.
Is there an idiots guide to IP address configuration that doesn't assume I'm a mathematician?