This is for for all you Zonealarm users that are having big troubles with the internet connectivity features with XBMC. I had added my xbox IP to the trusted zone and was still having problems with ICS. ICS worked perfectly when zonealarm was disabled but I wasn't too keen in leaving my computer unprotected. Finally I have solved the problem
I am assuming you have already set up ICS settings on your PC and xbox and ICS is working fine with zonealarm disabled.
Firstly start up ZA on the computer that you wish to share the internet and you'll see this screen.
Click the 'firewall'option on the left and you'll be at this screen.
Click on the advanced tab and this window will pop up.
Select the 'This computer is an ICS/NAT gateway'.
In Local address type in the IP of the computer.
Then click ok.
Well done hopefully you will now be able to access the net with Zonealarm enabled.