The registry is a central part of the Windows operating systems. The data in the registry is stored in more than one file, each file called a hive, hence the beehive icon for RegCompact. On Windows95 and 98 there are two registry hives, System.dat and User.dat. On WindowsNT and 2000 there are System, Default, SAM, Security and Software hives.
Most programs store their configuration data in the registry.
Over time, as programs are installed and uninstalled, the registry becomes fragmented. Also, when data is removed from the registry it's space is not reclaimed until more data overwrites the empty space. This has a significant impact on the performance of Windows and programs that make heavy use of the registry. Whenever you install software data is written to the registry so the software can later be uninstalled. What RegCompact Does RegCompact eliminates registry fragmentation by writing the data in the registry out into a temporary file.