The Windows Registry is not defragmented if you defragment your hard drive. This can be quite a problem for users who regularly install and uninstall applications on their system because uninstallers tend to “forget” to remove entries in the Registry during the uninstallation process. It’s astonishing that even the Registry of my brand new system [...]
Clean your Registry
Every passing day makes it more likely that the registry file contains obsolete entries that might make your system unstable. They could also slow down your system. It is a good idea to check obsolete entries in your registry from time to time and remove them to keep the unnecessary entries to a minimum.

