Install Snow Leopard without Leopard
Insert the Snow Leopard DVD into your DVD drive. Reboot the computer and hold down the Option key (newer keyboard call it "Alt"). After a few seconds, the screen will show your hard drive(s) and the DVD. Select the DVD, this will start the Installer.

If your previous installation of Leopard was corrupted by a failing hard drive, you will not be able to install Snow Leopard though (the Installer won't let you). In that case you must first replace the drive.