Miscellaneous Utilities : RU.Exe & NTRegOpt

 The hive file is ~18MB, and RU reports ~8MB. 

ru "HKEY_USERS\S-1-5-21-3455748456-1366409072-2060748656-1003_Classes"

Ru v1.0 - report registry key usage
Copyright (C) 2013 Mark Russinovich
Sysinternals - www.sysinternals.com

Values:       125496
Keys:         31951
Size:         8,465,316 bytes



dir \users\User\AppData\Local\Microsoft\Windows\UsrClass.dat /ah
 Volume in drive C is 
 Volume Serial Number is 

 Directory of C:\users\User\AppData\Local\Microsoft\Windows

04/28/2013  12:22 PM        18,612,224 UsrClass.dat
               1 File(s)     18,612,224 bytes
               0 Dir(s)  516,648,067,072 bytes free

In light of the recent introduction of the RU tool, Some may remember this utility called NTRegOpt...

http://larshederer.homepage.t-online.de/erunt/

I don't feel like testing it out right now, but if you do, just be sure to have the following before using:

1) Access to the system in an offline state.

2) Backup files for the original hive files. (which the utility creates)

It's been many years since I've played with the NTRegOpt utility; pre-Vista actually.  Past experiences showed marginal deductions in hive file sizes (<10%) with either no (or seemingly negative a few) impact to performance on systems.  Given the marginal impact, quit using it altogether after it rendered a system non-bootable.  (had to manually restore the original registry hive files)  But the boot issue was one out of many many uses/systems... an exception.

To note, I have in the past, not noticed any real performance improvements (even in booting) after using the NTRegOpt utility.

 Since the hive files are all cached, and disk transfer rates are much higher now, hive files often contiguous, I expect even less potential benefit with today's systems. 

The point of the RU utility is to help locate registry bloat, not to reduce hive file sizes.  From past observations unrelated to RU, can usually point to the "Interface," "TypeLib," "Classes," and "Installer" keys as representing significant portion of the registry.  But fiddling with data there can lead to problems that are very difficult to identify and track-back.  For instance, I had a problem with Vista Windows Explorer forgetting folder view settings, and had tracked the problem back to TypeLib entries imported for WordPerfect on an XP system; the problem resolved after reimporting the REG file, modified to delete the original imported entries.