Mörgæs is right, It's not like Windows where the registry gets full of cruft over time. But you can still benefit from doing some housecleaning with the Ubuntu Tweak janitor. See this link.
Install like this:
Open the terminal. Add the required repository with the command:
Update the software list with the command:
sudo add-apt-repository ppa:tualatrix/ppa
Finally, install Ubuntu Tweak with the command:
sudo apt-get update
After that, run it from the menu, or a terminal window or dash typing "ubuntu-tweak" depending on your flavour of Ubuntu and your preferences.
sudo apt-get install ubuntu-tweak