Windows XP ICS with SBS 4.5

Is there a way of manually overiding ICS?

I have an SBS 4.5 Network using the 10.0.0.0 network. All DHCP is handled by SBS.

I want to set up an XP Pro machine as an internet gateway. But I need to disable DHCP on the XP machine, assign it an IP address in the 10.0.0.0 network and still have the benefits of Internet conection sharing and Firewall.
I have set up IP Forwarding, but when I ping the internet, it times out.
Changing the IP addresses of SBS is not an option.
Any suggestions?
Mark