If you could try & clean the optical drive (canned air? maybe try cleaner disc too) that may help.
Also if you could find a second older PPC Mac with firewire cable to use, that may help in so
much as you may be able to use Target Disk Mode; to start your marginal Mac in that and try
to use the working second Mac as the install vehicle.
The system install disc has provision to erase & install; also one could attempt to re-set the
admin password given options in the utilities section of the installer. Or if you had access
to the later Leopard 10.5 retail install DVD, that may be useful in attempt to install new OS.
From my experience with iBook G4 (12-in 1.33GHz mid-2005 w/ 40GB HDD + 1.5GB RAM)
the computer does prefer the original (as shipped) Tiger 10.4/.11 over later Leopard 10.5.8.
And my original media also balked at allowing me the privilege of reverting; an erase &
install to all original applications included with Tiger, is where it's at now. Took 'effort' too.
Options can be a good thing; also clones on external capable enclosures can also be great.
I have a few of each; and a self-powered FireWire 400 hard drive (LaCie Porche design) with
oxford-type chipset that allows for PPC Mac OSX booting of the clones from that drive.
These are essentials, to continue the usefulness of vintage antique Mac hardware & OS X.