Essentially, a computer slows down because of an increased demand on resources. Over time we add: music, games, pictures, MOVIES!; and these begin to use up the finite resources of the computer. Before long, there are too many processes running, too little Memory and Disk Space available. Or, you have need of a Registry Cleaner, dust removal, or Spyware/Malware Removal... all of which can make your computer slow.