has been ranked as one of the most prevalent pieces of malware currently in circulation. Scanners as scareware Rogue Registry cleaners are often marketed with alarmist advertisements that falsely claim to have pre-analyzed your PC, displaying bogus warnings to take "corrective" action; hence the descriptive label "scareware". In October 2008, Microsoft and the Washington attorney general filed a lawsuit against two Texas firms, Branch Software and Alpha Red, producers of the "Registry Cleaner XP" scareware. The lawsuit alleges that the company sent incessant pop-ups resembling system warnings to consumers' personal computers stating "CRITICAL ERROR MESSAGE! - REGISTRY DAMAGED AND CORRUPTED", before instructing users to visit a web site to download Registry Cleaner XP at a cost of $39.95. Metrics of performance benefit On Windows 9x computers, it was possible that a very large Registry could slow down the computer's start-up time. However this is less of an issue with NT-based operating systems (including Windows XP and Vista), due to a different on-disk structure of the Registry, improved memory management, and indexing. Furthermore, versions of Windows prior to Server 2003 may fail to start up if the Registry and kernel files are unable to fit within the first 16 MB of memory. Slowdown due to Registry bloat is thus far less of an issue in modern versions of Windows. Conversely, defragmenting the underlying Registry files (e.g. using the free Microsoft-supported PageDefrag tool), rather than attempting to clean the Registry 's contents, has a measurable benefit and has therefore been recommended in the past by experts such as Mark Russinovich. (A form of defragmentation capability has been built directly into Windows since Vista.) The Windows Performance Toolkit is specifically designed to troubleshoot performance-related issues under Windows, and it does not include Registry cleaning as one of its optimizations. Undeletable registry keys Most Registry cleaners cannot repair scenarios such as undeletable Registry keys caused by embedded null characters in their names; only specialized tools such as the RegDelNull utility (part of the free Sysinternals software) are able to do this. Recovery capability limitations A Registry cleaner cannot repair a Registry hive that cannot be mounted by the system, making the repair via "slave mounting" of a system disk impossible. A corrupt Registry can be recovered in a number of ways that are supported by Microsoft (e.g. Automated System Recovery, from a "last known-good" boot menu, by re-running setup or by using System Restore).