OK
If all you need is port 80, then if you install WinGate, simply mark all interfaces as "external", which will firewall them by default, and open port 80 to allow access to port 80 on that machine.
If you want inbound web to go via WinGate's proxy, you'd need instead to bind the WWW proxy in WinGate to all interfaces, and have it set up as a reverse proxy to the internal WWW server. To do that
a) open web server settings in the WWW proxy
b) click Add to add a new site
c) on the general tab select "Reverse proxy request", enter the server and port (you'll probably need to get the web server to run on another port if it's on the same IP address)
d) on the hosts tab enter the sites that you want to serve, e.g.
www.yourcompany.comRegards
Adrien