Time to ask the obvious questions that I've asked so many times before. Did you burn the ISO file at a low speed? Did you check the md5 of the image that you downloaded? Have you run testdisk? Have you run memcheck? What version of Knoppix are you talking about and, if as expected it is the current version, had you been able to run older versions without problems?

If xmodule=nv resolves your problem, why do you not just do that? Lots of people use "cheat codes" to help Knoppix boot.