Some products such as registry cleaning utilities suggest that the registry needs regular maintenance or cleaning. However, serious issues can occur when you modify the registry incorrectly using these types of utilities. These issues might require users to reinstall the operating system due to instability. Microsoft cannot guarantee that these problems can be solved without a reinstallation of the Operating System as the extent of the changes made by registry cleaning utilities varies from application to application.
It has been a proven fact that you don’t need to clean the registry, it’s not like a defrag on your PC. You gain nothing from doing this. My experience of such programs over the past 20 years are that they are more trouble than they are worth, they cause problems, mostly with spyware or viruses. Even the Wikipedia article on this subject states “Registry cleaners are not supported by Microsoft, but vendors of Registry cleaners claim that they are useful to repair inconsistencies arising from manual changes to applications, especially COM-based programs.” Full details here ( http://en.wikipedia.org/wiki/Registry_cleaner)
This is the official Microsoft article on the use of such tools ( http://support.microsoft.com/kb/2563254) There is also a very good blog post by Mark Russinovich from Microsoft talking about this topic (http://blogs.technet.com/b/markrussinovich/archive/2005/10/02/registry-junk-a-windows-fact-of-life.aspx) Most good software will have a un-installer that will remove both the program files and the registry data.