You need to provide more information. For example, I took a Knoppix CD into work to show the sysadmin, it wouldn't boot on the particular machine we tried, that motherboard doesn't boot correctly from the Knoppix CD, but is fine with Windows stuff. I had problems booting a budget PC from the Knoppix live CD because it used shared memory for it's video. If when it fails it doesn't give some useful message, instead, drops to a blank (black) screen, do a search on this forum (or Google) to see if someone else has had success booting your particular machine with Knoppix, otherwise try and search on the message it failed with.

Check that the Knoppix CD is a good one, by trying it on a friends machine, I've had this problem before.

Cheers, Stu.....