I agree with all of the above, and I think a bad download (for some reason I seem to get a lot of bad Knoppix downloads that don't match the md5sum) is the most likely problem and a bad burn is the next likely canidate. That said, I have seen some older BIOSes that seem to be able to boot some CD's and not others. Do what the last poster suggested first, but if you're sure that your CD is good, test it in another computer. If it boots there but not in yours, there are still things you can do. If's it's not booting at all these might include BIOS settings, a boot floppy, or a boot manager like XOSL (fantastic free program). If it's starting to b0oot you might need some cheat codes. Post back here with moe information if you need help.

Also, if you need help and really hope to get it, you might want to put something meaningful in the topic. I expect many people are just skipping past this topic because the totle contains no real information at all.