Originally posted by cpjakes
Well, assuming that you have the original installer CDs, you should boot off of one of them (by loading the CD in, restarting, and holding the C key on startup). Run software restore with the option to wipe the drive completely. In OS X, you have to run Disk Utility from the Installer menu. In OS 9, I believe it is an option before you install.
This will erase all data and only install original software. Are you doing this because you're selling it or are you starting over with the machine?