Protocols / Routing :: LAN-Side Authentication At Router For Using Internet

Basically I have a win2000 server running as a VM in my computer with a secondary LAN address(Different subnet) which acts as a gateway for the other devices on my network (e.g. my wifi-router has DHCP relay set to my VM's address). The VM uses another network(internal NAT) for accessing the Internet though my computer. So far, I've managed to set up the DHCP and routing. Other devices on my network have access to the internet. But, since my computer has only one NIC, and even though the VM uses a secondary IP address, it's still on the same network as my internet connection. There are computers that belong to other people, who I do not want to give access through my router(even though their computers are picked up by my DHCP). I was given only one IP address for usage on my computer. This is why I wanted to add some kind of an authentication procedure(like some sort of PPPoE login maybe?)so as to prevent unauthorized usage my my connection. The question is, 'What are my options for the authentication procedure, and where do I start?' If my explanation was unclear.