Pop in a Leopard Install DVD. Boot into the disc, and once it's booted into the installation DVD then in your menubar click Utilities > Disk Utility. Click on your hard drive on the left side panel and go to the "Erase" tab. Name the drive what you want, and click Erase.
THIS WILL ERASE ALL CONTENTS ON YOUR HARD DRIVE.
It's completely safe, just make sure the things you need are backed up, your have everything you need backed up by Time Machine or another program.
Now you can install Leopard onto your hard drive, which is now blank and has NO operating system on it - you can install anything to it.
I don't see why you'd want to switch from SL to Leopard, but there you have it.
Enjoy