When you try to implement ICS, you receive:
Cannot enable shared access.
Error 783: Internet Connection Sharing cannot be enabled. The LAN connection selected as the private network is either not present, or is disconnected from the network. Please ensure that the LAN adapter is connected before enabling Internet Connection Sharing.
The computer that shares your internet connection, in Windows 2000, must use IP address 192.168.0.1. If that address is in use, you receive the above error.
You must change the ip address of the offending computer.