Microsoft released Regclean to scan the registry for invalid keys and values and then deletes them from the registry. After the release Microsoft Office 2000 and XP then Microsoft discovered that Regclean was actually deleting correct keys used by these products, and since then it is no longer supported by Microsoft.
This story can be applied to any registry-cleaner that have been created, all of them will at some time delete registry keys which are vital to a certain application, and stop that application from working. The speed gain of using registry-cleaners is non-existing, and trouble caused by registy-cleaners is numerous.
More Info MS KB299958