Take a look at the file "knoppix-cheatcodes.txt" on your Knoppix CD. If you type "knoppix noswap lang=en screen=800x600" at the boot: prompt you'll boot using kernel 2.4; if you use "knoppix26" instead of "knoppix" you'll start with kernel 2.6.

I suppose the next question would be, when you installed, did you make a boot floppy, and can you boot from it?