so wait.. you have a hardware firewall/router.. and from that router you have 8 pcs connecting (including this old one?). Depending on what you want to serve to the outside world (http? ftp?) you'll have to set up certain ports for port forwarding on your firewall/router (going to this "server" machine).
For http server software, I suggest the popular
Apache program.
Oh btw, I'm assuming your router/firewall is acting as a DHCP server (or you have static IPs correctly set up)