When attempting to upgrade from Tiger or re-install Mac OS X Leopard, you are prompted to erase your boot disk which is PGP Whole Disk Encrypted.
The Leopard installer requires a portion of free space following a system volume on your disk. When your disk was encrypted, that free space was used by a boot partition for the PGP BootGuard that allows you to login to your encrypted disk.
To upgrade from Tiger to Leopard or to re-install Leopard, your must first decrypt your disk. Your disk cannot be used by the Leopard installer if it is encrypted.
Use the follow steps to prepare the disk: