Umm... bru, your case isn't similar to the OP's
Michael has a functional machine with someone elses accounts on it.
Boot from the OSX CD, and choose Password Reset. This give you the ability to make your own administrative password for ThePreviousGuy. Now, boot the machine with ThePreviousGuy's password and using System Preferences: User Accounts (or something similar) create a new user for yourself, and give your new user Administrative priviledges. You may have to choose the option at startup to display the 2 users so you can select which one you will boot from.
Now boot from your new account.
Voila, you should have a spanky clean account, and you should be able to still use all the applications. You will be missing anything that was in ThePreviousGuy's personal Documents, Desktop, Music, etc. folders.
If you really wanted some of that stuff (hey, they were his personal tings - whattarya, some kind of voyeur?) you can log back on as ThePreviousGuy and move these items out from his Home folder to a common folder on the hard drive (not the Desktop)
Now you can reboot and log in as yourself, go back into System Preferences: User Accounts and Delete ThePreviousGuy.
If you run into issues with insufficient priviledges, then get yourself BatChmod from
www.versiontracker.com
Thanks
Trevor
CanadaRAM.com