The windows registry accumulates errors which slow down your computer over time. This occurs because your computer searches through the registry when retrieving information. Without the registry, your computer would not be able to run.
Your computer searches through the registry for your settings and preferences every time you boot your computer up. And each time you shut it down; it will record the changes you made while your computer was on. Obviously, the bigger your registry, the more time it will take for your computer to sort through...