I don't know anything about xp but with I've set up linux to use win98 internet connection sharing, win98 sets the "router" machine to 192.168.0.1, look at your NIC ip address, to see where it is set, then

So on your knoppix machine (using netcardconfig):
use a 192.168.0.X address (where X is 2-254 ie. if your xp machine is 192.168.0.1)
use 192.168.0.255 for your broadcast address
255.255.255.0 as your netmask
and use 192.168.0.1 as your gateway

HTH

~rock