Proxy Server 2.0 with one NIC

You mean you want to access both the Internet router AND 'internal network' using the same NIC ?
If you let through Internet traffic to your internal subnet, you're opening a huge security risk !
I recommend you get hold of another NIC that connects directly to your router.
That's not to say you can't assign 2 IP addresses to one card, if that helps ?
What exactly are you trying to achieve ?

If you do intend to go ahead with a one-card configuration, you could enable packet filetering - this can be difficult to setup if you're not quite sure what you're doing, in which case it's far easier to chuck in another network card :

http://support.microsoft.com/support/kb/articles/Q169/5/48.ASP?LNG=ENG&SA=ALLKB&FR=0

Well here is a trick which i have used at my work.
In your internal network assign some fake IP's for which one network computer can see another one. Then configure only one computer which is probably the domain controller to see the internet with the ISP's IP's configured (Static).If not static you can use the computer name as the proxy server name. Then configure the proxy on the domain and rout all the internal computers to the proxy server. this should work. I use Wingate Enterprise and it works perfect.
Hope it works.

To Cometj's

Can you give me some more details about you're answer