The Windows registry is a database which Windows uses to store all user and system related settings. Most Windows software also use the registry to store their settings. Most, if not all, of these software packages also do not remove their entries from the registry when uninstalled.

Needless to say, the windows registry tends to be in a mess most of the time.

As a Windows user, you also might have noticed Windows becoming progressively slower as you use it, installing and removing software, going about your daily business. One of the reasons for this is because the Windows registry tends to get bloated with stale entries and such, over time and since it is stored in the memory as long as the computer is being used, it slows down the whole computer.