If you have a 10.3.9 disc use that, if not try restore disc one. Put the disk in and reboot holding the C key on the keyboard. You should hear the CD drive whirring more than usual and boot will be slower. It should then give you a choice of languages followed by an installer. From the menus at the top of the screen choose Disk Utility, you should now be able to select and erase the hard disk.