Before you see Welcome to knoppix, there is an isolinux boot prompt
If you dont press any keys, knoppix boots within 30 secs, type

knoppix failsafe

at the prompt and press enter, this disables most of the hardware detection.

The knoppix CD is a compressed image, its around 2000M od data compressed
to fit on a 650M CD, using the El Torrito standard.
Its as iff the disk cannot be read, have a look in your BIOS and see if there are
any special boot requirements or configurations for the cdrom in your system,
there isn't on my system but its 4 years old.

Hope that helps,