This happened on both my wife's and my Macbook Pro with Leopard installed. So much for installing XP. Oh well.
Boot your mac with the installer disc, once booted, run Disk Utility (from menu bar). "Repair" your disk.
You can try a reboot and a disk repair within your OS, but this didn't work for me, the repair option was not available... hrmmm
The above worked on both machines for me.
James