Just run netcardcaonfig again as root, or on the KDE panel, there are configure options, look for netward card and use this
to reconfigure your ethernet.
Bear in mind if you are using knoppix 5.1 or above your ethernet card may not be eth0.
dmesg | grep eth

Will display the boot messages relating to ethernet.
You may find that if you have a firewire port this is eth0 and your ethernet card is eth1.
I would check this before configuring your card.

HTH