A windows registry is a database that stores settings and options for windows base operating system.
It stores information and setting for all of the hardwares, installed softwares, user settings on your computer.
Every time user makes changes on their Control Panel settings, make a new file associations, create a new or edit an existing system policies, or installed a new software, then all the changes stored in the registry.
Windows registry is actually a file that keeps a lot of settings.

The first windows version, which is windows 3.11 used a registry called Reg.dat. The next version, which is windows 95 and windows 98, used a registry called User.dat and System.dat. Then, Windows ME released with registry file called Classes.dat, User.dat and System.dat.
The newest version of Windows, such as Windows NT, 2000, 2003 & XP stores the registry files in this folder :
%SystemRoot%\System32\Config\, there are several files called Sam, Security, System, Default, Userdiff.
There’s one more file that separated in different folder, that is NTUser.dat. It is stored in the users profile folder under the Documents and Settings.

There are several benefits of windows registry :
1. The machine configuration separated from the user configuration. Everytime user logs into windows base operating systems such as Windows NT/XP/2003, user settings are merge with system settings.
This method will keep each user configurations, and at the same time they can also keep setting that can be used for all users.

2. Group Policy on a windows based computer network allows administrators to manage centrally all the programs and policy settings.
This feature can not be use in a home edition operating system. It can only be use in corporate edition operating system where a dedicated server exist.
But why ? It’s because the registry is accessible through a special Windows API that available to scripts and remote management using WMI (Windows Management Interface).

Besides the benefits, windows registry also has weakness :
Damage to the registry can cause windows system fail to boot. In a certain extreme case, the only way to repair is to perform full reinstall windows.
This is the main reason that we need to use registry scan and repair utilities.

By using a registry scan and repair utilities, we can easily make backup registry, and restoring is as simple as clicking buttons. We can also check on the registry whether it contains an entries that haven’t been deleted while we perform an uninstallation over a certain software. Once the utilites catch entries that no longer needed, we can clean the registry to increase the performance of the whole system.