Windows 2003 NAT on single interface

Hi.
I have a little bit of a problem configuring Windows 2003 to do NAT on a single network card (if that is possible). Here's the deal: i have a computer (A) connected to a gateway (G) providing Internet. The computer A has only one network card. I want to use the A computer as a gateway for another computer (B) through that single network interface.
I have activated in Routing and Remote access NAT/Basic Firewall and added Local Area Network for NAT-ing but it doesn't work.
What's the catch? Isn't it possible? In Linux a single line in the firewall solves the problem. Please HEEEELP!