A registry cleaner is a class of third-party utility software designed for the Microsoft Windows operating system, whose purpose is to remove redundant items from the Windows Registry. 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. The effectiveness of Registry cleaners is a controversial topic. The issue is further clouded by the fact that malware and scareware are often associated with utilities of this type. Advantages and disadvantages Due to the sheer size and complexity of the Registry database, manually cleaning up redundant and invalid entries may be impractical, so Registry cleaners try to automate the process of looking for invalid entries, missing file references or broken links within the Registry and resolving or removing them. The correction of an invalid Registry key can provide some benefits; but the most voluminous will usually be quite harmless, obsolete records linked with COM-based applications whose associated files are no longer present. Registry damage Some Registry cleaners make no distinction as to the severity of the errors, and many that do may erroneously categorize errors as "critical" with little basis to support it. Removing or changing certain Registry data can prevent the system from starting, or cause application errors and crashes. It is not always possible for a third-party program to know whether any particular key is invalid or redundant. A poorly designed Registry cleaner may not be equipped to know for sure whether a key is still being used by Windows or what detrimental effects removing it may have. This may lead to loss of functionality and/or system instability, as well as application compatibility updates from Microsoft to block problematic Registry cleaners. The Windows Installer CleanUp Utility was a Microsoft-supported utility for addressing Windows Installer related issues. Malware payloads Registry cleaners have been used as a vehicle by a number of trojan applications to install malware, typically through social engineering attacks that use website pop-up ads or free downloads that falsely report problems that can be "rectified" by purchasing or downloading a Registry cleaner. The worst of the breed are products that advertise and encourage a "free" Registry scan; however, the user typically finds the product has to be purchased for a substantial sum, before it will effect any of the anticipated "repairs". The rogue security software "WinFixer" including Registry cleaners