How to clean Window Registry

Before cleaning up your Registry file in your windows ,you should know what is Registry and why they are used.The Registry is a database used to store settings and options for the 32 bit versions of Windows. It contains information and settings for all the hardware, software, users, and preferences of your computer.

Structure of the Registry

HKEY_CLASSES_ROOT - This branch contains all of your file association mappings to support the drag-and-drop feature, OLE information, Windows shortcuts, and core aspects of the Windows user interface.

HKEY_CURRENT_USER - This branch links to the section of

HKEY_USERS appropriate for the user currently logged onto the PC and contains information such as logon names, desktop settings, and Start menu settings.

HKEY_LOCAL_MACHINE - This branch contains computer specific information about the type of hardware, software, and other preferences on a given PC, this information is used for all users who log onto this computer.

HKEY_USERS - This branch contains individual preferences for each user of the computer, each user is represented by a SID sub-key located under the main branch.

HKEY_CURRENT_CONFIG - This branch links to the section of HKEY_LOCAL_MACHINE appropriate for the current hardware configuration.

Editing RegistryEditing your Registry is higly risk so don't do this without learning .

Clearning Registry 
You can clean unwanted Registry file in your windows using software here is the list of free software to clean up registry file
Free Registry Cleaner.
Eusing.
CCleaner
Then main think before Clearning Registry ,you should back your registry file .