The registry is a database where the settings for Windows and all of your installed software are kept. There is one copy of the registry for each user in hidden files called ntuser.dat located in the root of each user account folder on your Windows drive.
The Windows registry can be manually edited using Registry Editor. To open this panel, type registry in the Start menu search box, and then select Registry Editor from the search results that appear.
Occasionally, you may have to change or remove a setting in the Windows registry. You should always be very careful when you do this, because changing the wrong setting in the registry can cause Windows 7 to become unresponsive or unable to boot.
The registry is separated into five different sections.
• HKEY_CLASSES_ROOT You should not change these settings. They include essential
Windows system settings along with other things such as file associations.
• HKEY_CURRENT_USER These are the custom settings for the currently logged-in user. These will include settings for Windows and installed software. These are the most commonly changed settings.
• HKEY_LOCAL_MACHINE This section is for general Windows and software settings. You may need to make changes in this section.
• HKEY_USERS This section is for general controls for user accounts; you will not need to change these settings.
• HKEY_CURRENT_CONFIG These are additional settings related to your current configuration; you will not need to change these settings.
There are only two situations when you will really need to change or remove settings in the Windows registry.
• When you are following specific written instructions on dealing with a problem, or tweaking advanced (hidden) settings in Windows 7
• When you are removing settings left behind by an uninstalled program that are causing problems with Windows 7 or other programs
The most common things you will need to do are create a new setting or change an existing one. You can do this by navigating to the correct place in the registry (you may be following specific instructions from a website or manual, which will guide you to exactly the right place in the registry) and right-clicking either a blank space to create a new setting or an existing setting to change it.
Source of Information : Microsoft Press - Troubleshooting Windows 7 Inside Out