from http://www.knoppix.net/docs/index.php/FaqUsing:

Q: After using KNOPPIX, my Dell computer does not start!

A: This is an issue with the Dell BIOS. Simply pull out the power cord from your computer for a few seconds, and your computer should start again. You can also try passing "apm=off" or "apm=real-mode-poweroff" to the kernel during startup. apm=real-mode-poweroff is better because it keeps other apm-features (advanced power managment) turned on.
Hope it helps

Alessandro