Assuming your BIOS is set up to poll the cd/dvd drive in search of a bootable os, before polling the hard drive, then put the cd/dvd in the drive and reboot the machine. It should then boot Knoppix instead of whatever os is installed on the hard drive.
If you have followed the instructions above and you get the popup message instead of the Knoppix boot prompt, it's time to suspect either that your iso download is corrupt, or the burn to cd/dvd is corrupt. Check the md5sum of the iso you downloaded against the md5sum of the iso at the Knoppix website.
If you see a blinking cursor around midscreen as well as the popup message, try entering something like "knoppix lang=us" without quotes to see if it will boot properly.
Last edited by bigrigdriver; 04-03-2009 at 10:08 PM.