So, it’s ok to totally wipe away EVERYTHING on the computer, right?
Put the OS X CD in the computer and re-start it holding the “C” key.
When the installation screen comes up click the first ‘ok’ button (or whatever it is) and you should see a normal Apple-menu bar across the top of the screen. Find the ‘Disc Utility’ in there and select the system hard drive. Chose the ‘Erase’ tab, give it a new name and set it to “Mac OS Extended Format” with journaling on. I THINK that there should be an advanced option window you can open where you can tell it to write zeros to the whole hard drive first. That might take an hour but it’s your safest bet.
Once that’s done go back to the installation part of the CD and do everything it says to do. Once you’re up and online, run the System Update in the Apple menu. Restart when it’s done and check it again. Do that as many times as you need to until no updates appear in there. THEN install your applications from their original CDs and run System Update AGAIN and restart AGAIN and do that until nothing more comes up.
Then you can finally start customizing your system prefrences to your liking.
That’s about the most straightforward, safest, leave-nothing-behind kind of way to do it!