The first thing you can do is to check the cd's integrity (a check to see if the cd image is correct) by typing 'knoppix testcd' and hit enter (takes a view minutes).
If the integrity is oke you could try one of the cheatcodes to see if it will boot. The cheatcodes list can also be found on the downloadserver (knoppix-cheatcodes.txt). Try the cheatcodes 'knoppix noapic', 'knoppix noapm' and 'knoppix failsave'.