Sounds like it may be time to learn to use ethereal (included on the CD) to "sniff the connection" and watch what the NIC and router are saying to each other. That would tell you several things, like does the computer see the ARP packets that should be there every few minutes? Is there any response at all when the computer tries to send a packet? You could also watch the DHCP exchange and see where it goes wrong (by comparing it to an exchange on your good Knoppix system).