Sometimes finding answers to the dirt simple questions is the most challenging.
You want to boot Knoppix. You have an old machine on which the ATAPI CDROM has died. You have a USB CDROM lying around you could use, but oh wait, the BIOS on your old machine does not support booting from a USB CD.
What do do?
Assuming you have a floppy drive in your old machine, boot Knoppix on another machine and run the "mkbootfloppy" script from a terminal window. This script will create two floppies, a boot and a ramdisk.
Plug your USB CDROM into your old machine and boot using the two floppies you just made.
The Knoppix boot floppies will automagically find the Knoppix CD in the USB CDROM and boot from it.
In my case I used Knoppix 3.7.