Reformat and Partition Your Hard Disk
Back up everything before you begin. I cannot be held responsible for any files you lose if you insist on not backing up everything you need! Proceed at your own risk.
You cannot reformat the disk that is running the computer, so you have to insert a system CD and let the system on the CD run the computer. What you are about to do is this: You’re going to insert a system CD and start up from that CD, reformat the drive (destroying every iota of data on the entire hard disk), then reinstall everything like it was when it came from the factory. Follow these directions:
This is the Disk Utility that will format and partition disks for you. You can use it to erase a Zip disk or a partition (Disk Utility is ...