It's rather strange that Knoppix can't boot even in failsafe mode. Did you confirm the MD5 checksum before burning the ISO, burn it at low speed and do a verify option when burning? You are using a 700 meg CD and not a 650 meg disc, correct? If all of these can be answered yes (particularly the md5 question, no fair ignoring it just because you don't know what md5 is), then try booting and type memtest at the prompt to run memtest86 and confirm that the computer's memory is OK.