The Auslogics Disk Defrag software is a popular hard disk defragmentation tool and many users who like it will probably also find the Registry defragmentation tool useful. The Windows Registry is not defragmented during hard disk defragmentation because it is not possible to defragment files that are open for exclusive access.
Auslogics Registry Defrag reduces the size of the Registry by defragmenting it which has the effect that the Registry will take up less memory when Windows is running which does increase system performance including boot time. The tool initially scans the Windows Registry and displays a report of the fragmentation level of the Registry afterwards.
It is recommended by the application to close all open applications to maximize the effectiveness of the Registry scan. Once started the mouse cannot be moved outside of the program’s interface and even the usual ALT TAB or Windows keys are disabled. The user has the option to quit the application at any time or proceed with the defragmentation which requires a system restart.

As you can see in the screenshot above the tool was able to cut the size of my Registry by more than six Megabytes from 34 to 28 Megabytes which is a size difference of 18%. It cut down my boot time by two seconds.
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Just as with disk defragmentation consolidating free space in the registry creates future fragmentation.
Check the whitepaper here..
http://files.diskeeper.com/pdf/HowFileFragmentationOccursonWindowsXP.pdf
Like many things it makes so much sense once you know.