Since the Windows Registry was first introduced in Windows 95 all hardware and software configurations were gathered and stored in the registry.  However the continual use of a PC and the constant modifications done within the system can “build up” bad data in the PC like broken or orphaned registry values.

Orphaned registry entries tend to side swipe Windows and slow down the startup process or possibly cause other types of errors that can completely shut down the Windows operating system.  A good Registry Clean Up tool also cleans out and fixes other types of errors other than just the orphaned type of registry entries like, viruses, and invalid keys.  From the link above check out some reviews of registry clean up tools available to you.