Buy a router (they are cheap) so you don't need to share connections on each PC. Each machine is connected directly to the router and the router is connected to the internet.

I don't believe the current configuration will work without complex configuration. More information is needed about how you've set up IP addresses and routing tables.

It makes more sense to put 2 NICs in the W2k3 machine and use its RRAS capabilities in lieu of a router. There are lots of help documents for setting up RRAS. MS Technet has a lots of white papers on the process as well.