How to install Leopard over Lion - problems

On my Macbook Air I want to return from Lion to Leopard. I use the original Leopard CD but I get the message that I can't run this installer over this system. I found on the forums here that I have to erase my disk first from Disk Utility. But this option is greyed out. Obviously! How would you erase the system being logged into it. You need to run the Disk Utility from the Installer but the Installer won't start! Now what?

Any suggestions?