What you need to do now, every so often, is "repair permissions" or at least "verify permissions". When you use your "Disk Utility" in "Applications" you can choose these options and watch as your computer analyzes itself and its important files. Defragging is definitely a dying need since OS X takes care of these stray bits so well. OS X even handles RAM allocation automatically! (something you used to have to set yourself on a mac, which for dyed-in-the-wool macfanatics wasn't such a bad thing...)
