The BIOS screen is most often reached by pressing (and sometimes holding) DEL as booting starts, although some systems are indeed different.

I can't account for why the CD will no longer boot, but I also can't understand how you could zip an entire Windows partition with the relatively small Knoppix RAM disk and unwritable disk partitions. The bestI can offer is still to reset the BIOS, then be sure that the CD is in the boot order before the hard disk, and try to get the Knoppix CD to boot, but I have to say that the feedback we are getting sure doesn't make much sense.