I have several older Digital machines that are perfectly capable of booting from cd but simply ignore the Knoppix disc in the cdrom drive for some reason. Booting w/ a Knoppix boot floppy fails as well. What does work is SmartBootManager

You can easily create a floppy from windows. Open up a DOS prompt and cd to wherever you downloaded sbminst.exe. Run it like so:

sbminst.exe -d 0

That will put SBM on a floppy for you and should (hopefully) let you boot from cd. It worked for me anyway.