You should just be able to set the network setting on the Win2k box to DHCP. It should then pick up an IP address of 192.168.0.x where x is a random ip address other than 192.168.0.1 which the XP box should have set itself to when you enabled ICS.
Check in Internet Explorer to make sure you have all the boxes unchecked and boxes empty under the LAN Settings button in the connections tab.
If your IP addresses are set to anything other then 192.168.0.x then ICS probably isn't working properly on the XP machine. I've never managed to get it to work on any address range other than this.
If both ip addresses seem ok, then reboot the XP machine first, leaving the Win2k one off until XP is fully loaded. Then boot the Win2k box to make sure it picks up the correct dynamic address from the XP machine.
If it still doesn't work, as a last resort, try putting the second NIC in the Win2k machine and the cable modem on there instead.