How to share a PPPoE internet connection?

My ISP provides me internet through a network cable and the PPPoE protocol. I set up my internet connection using pppoeconf and it's working nicely. The thing that I didn't manage to do is share this internet connection to another computer running Windows XP SP2 through my second network adapter. The one which connects me to the internet is an nVidia onboard network controller, and I have another SMC 10/100 PCI network adapter which connects me to the other computer.

I followed the guide here: http://www.ubuntugeek.com/sharing-internet-connection-in-ubuntu.html but with no luck. I read the ubuntuforums.org posts where someone suggested to use Firestarter. Firestarter says it can't start the firewall at all (after I run the wizard for the internet connection sharing).

The truth is that I am a little confused because I had the network interfaces in my system: eth0 (the onboard controller) and eth1 (the PCI network card), and after I set up the Internet connection I also have a ppp0 interface. Firestarter won't start the firewall either if I chose eth0 as the internet connection interface or ppp0.

I think I have to select ppp0 for the internet interface, because ppp0 is basically a virtual device which uses the eth0 physical device.

My question basically would be how on earth can be set up an Internet Connection Sharing preferably easily, but at this point I'm willing to try anything? The Firestarter solution would have been easy if it worked.