Trying to install OS X v10.5 Leopard on iMac G5

Trying to install OS X v10.5 Leopard on iMac G5 PowerPC:

Using Family Retail Pack DVD (it's clean/no scratches etc.), have used the very same DVD before on this particular iMac and was successful in the past.

Checking Installation DVD stage goes ok, installation begins, but DVD gets ejected and Mac OS X Installer screen freezes with spinning disk icon.

Installer Log keept listing the following error messages:

1. localhost installdb(173): _PubToPrivRef (): unable to find db handle 1.
2. localhost istallldb(571): - (IFDBD_SchemaA closeNewReceipt): database error (1): SLQ logic error or missing database

Forced shutdown and switched iMac back on, first a grey screen with a purple folder appeared (in centre of folder a question mark was flashing and alternating with the Finder icon).

Then the screen turned blue with spinning disk icon.

Any suggestions would be greatly appreciated!