When knoppix is installed, it's just a little less secure than debian by default. ( due to many more programs installed )

But you can increase security by adding firewalls, kernel patches etc.

If you run from the cd on a standalone computer with no harddrive, i'd say that would be pretty damn secure. Just reboot, and you're back to a clean install.

It mainly depends what you mean by "Secure" and what you intend to do with it.