You should clean your Registry as part of troubleshooting a software problem. The problem could be specifically with Windows, or it could be a third party software application or game.
A Registry cleaner scans through the Windows Registry and removes information that's no longer needed. As Windows and your other software operate, they should automatically "clean up" after themselves. But they do not always, and over time electronic residue and debris build up. The Registry grows bloated. If this gets bad enough, it could start interfering with normal operation.
The Registry concept was introduced in Windows 95. Part of the reason was to have one gigantic centralized place to store software information. This made programming easier and helped Windows operate more efficiently. Theoretically.
In practice, it's taken a while for Microsoft to get the Registry operating properly. Some would argue there are still flaws in the Registry concept itself. But typically, Windows XP machines are pretty Registry-stable. If you think you may need to clean your Registry, here are reasons and situations that could indicate it's time:
o A program uninstall had problems, and the software was not uninstalled properly.
o After installing or uninstalling a program, Windows starts crashing or acting funny.
o You've installed and uninstalled lots (and lots) of software over the years, and are noticing more and more errors when doing so.
o You're running Windows ME, Windows 2000 or older software, and are noticing problems like those above. These versions of Windows were less efficient at Registry management.
o Also read about the ScanReg and System Restore tools, as both of them are programs allowing you to save "snapshots" of your registry at different times. You can then reload old versions as needed to restore your system back to a (pre-problem) state.