Internet Connection Sharing Problem

I have two PCs in my current setup. The first (PC1) is connected directly to the internet via USB modem, and shares the internet connection to PC2 via ICS, connected by ethernet.


With "Internet Connection Sharing Support" turned on under Firewall > Advanced, and with the firewall turned OFF the second PC can access the net. However, with the firewall turned on, I can't get anything. Not even ping replies from PC2 to PC1, though the other way around is fine.


There is no protection on PC2; no firewall or antivirus. The problem exists with the BitDefender firewall on PC1. ICS is allowed on the internet connection.


Any ideas? I've set rules to allow everything to and from PC2, but that just about allows ping, and doesn't allow ICS.