I think you will find that most ISP's say that they don't support Linux. I personally find this lame, but back to your issue. from what i have read on google, you shouldn't need to do anything for knoppix to work on comcast..
Taken from this page :-
http://www.johncon.com/john/knoppix/
"As an added note, Knoppix Linux requires no configuration when used with the Comcast service-the CDROM boots and auto-configures the computer for Internet access, all automatically."
So maybe your issues are not with your ISP but with your hardware?
if you type 'ifconfig' in the command prompt, do you see an entry for 'eth0' ?
If not, it could be a driver issue, if its there, try running 'dhcpcd eth0' and see if you get an IP address