LAN to LAN over internet

It sounds like you are looking for a proxy or gateway solution not necessarily a VPN.

If you want to enforce a policy using the 2003 server for Internet traffic you will need to either block all other hosts from the Internet by using your router's configuration tools or setup the 2003 server as a router and connect only it to the Internet. The first method is usually easier to implement but can cause some user confusion.

Depending on your needs you may want to also setup automatic proxy configuration.

I know this answer is somewhat general. I hope I covered it with enough information to get you pointed in the right direction.

Normally VPNs are used to connect hosts and networks privately across public networks. Proxies are used to enforce network policies on traffic crossing a network boundary. Gateways (routers) are used to actually pass traffic across boundaries.

You need to get your win2k box (3.) to do Network Address Translation with RRAS. It is possible, but painful.

I would personally either replace 3b with a DSL modem that will terminate & NAT the connection, or upgrade 3. to Windows Server 2003, which does allow NAT without so much pain.