Frankly, I'm not too familiar with Win2000 ICS (I use Wingate on a stand alone NT machine), but if you just plugged the ADSL modem in with ICS already installed, where it had previously been using the modem as its internet connection, then that would be my guess for a first stab at the source of the problem. I'd uninstall ICS, restart the machine, and then reinstall and set it up again from scratch on the server and each of the clients.
Perhaps you've already done that.
When you ping from a client machine, can you ping using a name (eg ping www.bigpond.com) or only to a known IP address (eg ping 203.210.11.18 -- except I made that number up)? What is the exact error message the ping gives?
