Hey thanks for reading this I hope you can help,
I have a Compaq (HP) laptop with a National Semiconductor Corporation DP83815 Ethernet Controller. I am booting off of several different live CD's and having the same problem with each. I know the NIC works in windows but when I boot into linux the NIC acts if it is not turned on. I run a ifconfig -a and it shows me the NIC is there I have added the route for the gateway and all is correct. The NIC just acts like it is not turned on because there is no light on the back of the card .

Can anyone Please help I would like to get this working so I can use it as a proper Diagnostics tool outside the office.

Humm.... The media used is fully functional in windows as well? Try running

dhcpcd eth0

See if the dhcp server your hooked up to (assuming your on a network with one), assigns you an IP address... My guess for now if the media is working fine. The only situation I have found where the NIC looked "Turned off", was in Windows when I had it disabled in the device manager (On an IBM Thinkpad). Hope it helps a bit.

I have discovered it will work just fine if I hook it to a network running DHCP but if I try to manually set up the IP so that I can run on a different network one that does not support DHCP the NIC shows no life. It is just like you said the NIc appears disabled or something...