Mic,

Here's what knoppix does:

The Knoppix iso is downloaded and burnt to a CD

If you can get your PC to boot up from CD, you can use this to load knoppix without touching your windows installation.

Play around a bit, check out all the software. You will be able to see your hard drive(s), CDs etc.

When you are ready to install to the hard drive you can run the knoppix-install script which will give you a Debian install.