Can you run live USB Linux after boot from CD?
 

For an older computer that won't boot off USB (like the live knoppix/ubuntu, etc) , is it possible to have it boot off of a special CD that will tell it to run the operating system off an attached USB thumbdrive?

It's doable (but I've never done it). Perhaps this will help

Plop and a copy from slitaz can allow a non-bios system to boot to a usb flash. It might be that a very old cd will work also. I think it is called smart boot manager.