You should backup all of your data before doing any disk work. It is ok to fix the startup volume with your Leopard disc, even if the current OS is Lion.
To do that, put the reinstallation dvd in the drive and reboot the Mac while holding down the C key. It should then startup into the Leopard installation. Select your language. When you get to the Installer screen, click the Utilities menu and open Disk Utility. From there you should be able to repair your startup volume.