It sounds like you're trying to use the server as a router (internet gateway). If so, you should be using ICS. You establish the Internet on one connection, then enable ICS on that same connection. ICS will automatically configure the other connection for you, and provide a DHCP server as well.
You don't want to use XP as a "server" it has a connection limit of clients that can attach to it.
If you buy a Router it will come with instructins on how to configure it. Should be easy as plugging the modem to the WAN port of the Router, and then hooking up the router to your existing network switch. Set your computers to use DHCP on the network cards if you have them for static. Router will share internet connection to all the PCs.