You have to troubleshoot this from a networking perspetive. Forget the way you connect to the internet and what systems are in place, for the moment.
The fact that the thin clients can connect, indicates that from the 2003 server itself, you have internet connectivity. If the XP systems do not, check their IP configuration. Their IP settings should reflect thte following:
Ip on the same segemnt as the internal NIC of the 2003 server, same subnet mask, and the XP's default gateway should be the internal NIC of the 2003 server. If that is configured correctly, then the next step is to validate if the ICS configuration
on the 2003 server is set up correctly.
By the way, I would recommend that you remove ICS (since you are running 2003 server) and enable Routing and Remote access. Its a more reliable service and you have more configuration options avaialbe to you.
Aside from this advice to help you remedy your current situation, I would tend to think that a better design would be to not use the 2003 server for internet connectivity, but rather move back to the broadband connection, connect to the internet using
a consumer or enterprise entry router, then simply allow all of your clients (2003 server and xp clients) to use the router for access to the internet.
Visit:
anITKB.com, an IT Knowledge Base.
