I've told you so many times that using your computer to share your Internet service - using ICS - is not a good idea. My reasons center around the needless complexity, and load, placed upon the server.
Now we have a new reason to avoid ICS. This week, ICS is being used as an attack vector. NetworkWorld: New Windows attack can kill firewall points out
The code, which was posted on the Internet early Sunday morning, could be used to disable the Windows Firewall on a fully patched Windows XP PC that was running Windows' Internet Connection Service (ICS).
So, if you're running ICS because you have 2 computers, and you think that Windows Firewall protects you, think again. Even if your computer is fully patched, if you have ICS behind WF, you are at risk.
Perimeter protection, aka a NAT router, is the only real protection.