From YL Computing:
WinUtilities Registry Defrag analyzes the structure of your registry and rebuilds it completely from scratch. thus removes any slack space that may be left from previously modified or deleted keys. The registry is defragmented, and any structural errors are corrected. By defragging your registry, it will create a more linear structure maximizing application response times and registry access times, saving memory, and enhanced boot-up times. This can greatly improve system performance and to avoid system crashes and the loss of data.