You can do that, no problem.

Just boot knoppix, wait until it all loads, open up a Terminal and type

sudo knoppix-installer (I'd pick the Debian install, every time)

and follow the prompts.

Just make sure that you write to the correct HD.

In Linux speak hda is the first drive on the primary IDE channel (primary master), hdb the primary slave, hdc the secondary master and so on. Note that for the purposes of identifying drives, any of these could be a CD device. Windows likes to be on the primary master.

The usual caveat: back up anything really important.