I assume from your statement, you are using static IP addresses. A better solution is to use DHCP to assign the addresses. Getting the MAC address and/or controlling who has access to Internet is then pretty simple. If you're using Linux for the DHCP server, the MAC address and corresponding IP address can easily be obtained from dhcpd.leases.

I suspect there is a similar way under Win2000 to get the MAC addresses and corresponding IP's, but I have no first hand knowledge.

Depending on your firewall / proxy server, I suspect the same information could be obtained from the logs there.