Thanks for the replies.
I ran the Apple hardware diagnostic disc, everything came out OK. The OS disc is a retail Tiger one. I also took the PowerBook to the Genius Bar at the Apple Store, they did exactly the same thing I did, told me to do a clean install--but to no avail. Machine won't boot from Tiger, but boots fine from 10.3.
I was hoping there might be a simpler solution--I don't feel confident after looking at the procedure to replace the hard drive and I'd rather not sink any more money into an outdated machine.