Microsoft Proxy Server 2.0 on Windows NT 4.0.

It's not our only firewall, it's actually a second stage firewall behind our Linksys NAT router/firewall. I wouldn't expose a Windows box directly on the net unless I had to.

(Note that I -DO- have one windows box running as a test/development web server which is exposed directly to the internet, but it's physically separated from the rest of our network and is on its own independent DSL line.)