Insert your Panther installer CD in your external drive. Restart your PowerBook, while holding the option key. Wait for your installer CD icon to show in the blue 'option-boot screen'. Click on that icon, and then click on the right-facing arrow. After you boot to the installer, you can choose Disk Utility (from the Installer menu), and erase your hard drive. Or, (guessing that you want to re-install Panther), Click the Option button at the screen when you select the drive, and choose the option to Erase and install.
Serendipity is a lucky guess !