I had a host machine I wanted to enable ICS on. First I realized that doing so was not possible until I enabled the Windows Firewall. Once I enabled the firewall and set up ICS, I noticed due to group policy I could no longer disable the firewall. Also, any ports I tried to open seemed to be ignored. Although nothing seemed to be configured when I used the mmc snap-in to view local computer policy, when I checked the registry I noticed several policies set there in HKLM (such as disabling AllowLocalPolicyMerge). I was able to remove the policies from the registry and my open ports worked, but they were eventually re-added without my input.
The network I am sharing the internet from is an "unsecured" wireless network with an authentication page, is it possible that this is causing those policies to be set? Did ICS set those policies?
When you go to the properties of the ICS enabled adapter and go to the ICS settings it takes you to a tab called services where you can add and remove "services running on your network that internet users can access". Is this related to the windows firewall?