XP Pro Won’t Share Internet Connection

Running XP Pro SP2. One NIC for ISP input from cable modem using pref & secondary dns servers. Can connect to ISP ok. Second NIC for home network, but other computers can not see the “sharing” computer or get to the internet. I have tried different NIC’s, uninstalled and reinstalled software and run/re-run the internet setup wizard, no luck. DHCP service is running. The second NIC has been assigned an IP and subnet. When I remove this IP/Subnet the other computers can “see” each other, but then there is no internet connection. I have repaired the winsock, still not fixed.