To do a clean install you need the installation DVD's off OSX. When you have them you need to insert the first disk and shut down the Mac. Now you hold down the C key and start-up the Mac again. The Powerbook will start-up from the installation DVD. Just follow the steps on your screen to re-install OSX. Make sure you have selected "archive and install" at the "customize" menu.
After OSX has been installed, you should be able to find some document's with the owner's name or e-mail adress.
Hopefully this solved your problem.
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