Are you using the white Snow Leopard retail disc? You need to be using the gray discs that came with your machine - NOT an upgrade disc. If you don't have the discs that came with your machine you can contact Apple, give them your serial number and for about $20USD they will ship you the machine-specific discs which shipped with your model.
If you are using the gray Leopard DVD that came with your Mac, you should be able to boot from it, erase the hard drive and re-install the system. Then you can use the white Snow Leopard disc to upgrade your system. Just insert the gray disc, hold down the option key whilst booting and select "Disk Utility" from the Utilities listed. Erase the drive and then you should be able to install Leopard (if, indeed, that's what's on the gray disc and the system that shipped with your model).
Good luck,
Clinton
MacBook Pro (15-inch Late 2011), OS Mavericks 10.9.4, 16GB Crucial RAM, Crucial M500 960GB SSD, 27” Apple Thunderbolt Display