Reinstalling OS-X
It sounds to me that you didn't go into Disk Utility before commencing the install process. Put the install CD in and boot up to it. Be sure to hold down the C key during the boot so it looks to the CD to boot up. After it boots up and it is ready to install, don't continue yet. Go up to the pulldown menus and start the Disk Utility. Highlight your hard drive in the left pane then select erase on the right. On the erase page there is another button closer to the bottom called security options. Go there to set how you want to erase your disk. I'd imagine that you want to format using Mac OS Extended (journaled) so set the format to that then let her rip. Unless you have secret data on your drive, a lot of people just write zeros once and everything is gone.
I just reinstalled OS-X on my Powermac using two 250 GB drives. I software striped them together for raid 0 and just used the write zeros once option. That's a lot more info than you need to do your Powerbook but I want to stress that writing zeros more than once may be a waste of time to you. Writing zero once takes a little while, writing zeros seven times takes seven times longer and so on. You get the picture about 35 times? Yeah, I thought "yikes" too. You probably don't need 7 times or 35 times unless you are DOD, CIA, NSA or NASA!
When you are in the security options page it should should explain what each level does and you can decide what to do.
Don't forget that a 100 GB disk will never have 100 GB available. Formatting requires some space but I can't remember how to calculate it. However, you should get back to what you had when the system was new.
So, just a reminder, don't forget to go into Disk Utility to erase/format before you proceed with the install. You'll do fine!
