Compress NTs Registry

NT’s registry is implemented as a database. It shares many of the
characteristics including fragmentation as items are added, removed, or changed.
In particular, when items are removed from the registry, there is no cleanup
mechanism to recover the now unused space in the database. Microsoft released RegClean to clean up some of the debris. A alternative
to Microsoft’s RegClean is the freeware Regcleaner .Mark Russinovich has released PageDefrag which can defrag the page file and registry hive
files.

Fragmentation can become significant over time on a busy system. Use either
tool with caution. There is no tool to compress the registry, to recover lost
space and reindex the fundamental database. But there is a method to achieve
this result. Backup the registry and be prepared to get back to where you
started if things go wrong.

• Update the %systemroot\repair directory using the
  command
  

  rdisk /s-
  

  rdisk will update repair data including the default,
  software and system hives. The
  repair version of the hives is compressed and reorganized. It is not an image
  copy. Adding the /s parameter gets the sam and security hives. The dash, -, instructs rdisk to not make a
  floppy disk copy.
  

  
  
• Expand the registry hives to a temporary location:
  

  expand %systemroot%\repair\default._
  C:\temp\reg\default
  expand %systemroot%\repair\sam._
  C:\temp\reg\sam
  expand %systemroot%\repair\security._
  C:\temp\reg\security
  expand %systemroot%\repair\software._
  C:\temp\reg\software
  expand %systemroot%\repair\system._ C:\temp\reg\system
  

  A comparison of the files sizes in %temp% and
  %systemroot%\system32\config can reveal significant
  differences in size. If the sizes are close, the hive in question did not have
  much lost space to recover. Usually the software hive has the greatest space
  recovery.
  

  Replace the version in %systemroot%\system32\config
  with the newly reindexed version in %temp%. NT keeps the
  hives open and locked. You can not simply copy the new version over the old
  versions. To get around this problem, you need to make the copy when NT is not
  loaded. For a FAT-based installation, bootup using DOS and make the copy using
  the DOS COPY command. For NTFS-based installations, I recommend using a recovery
  or backdoor copy of NT. Install NT on the PC in a different directory. Boot
  under the secondary NT and copy the hives for the inactive primary NT
  installation.

There is some risk in this procedure. But as the
hives get large with a high amount of dead space, performance suffers. If your
server or workstations gets slower and slower for no apparent reason, you may
get the performance back using this approach.

Do not attempt if you are not in a position to recover back to the starting
point.

A safer and easier alternative is RegCompact
.

Microsoft presents three methods to compress a bloated SAM on a PDC / BDC. Scary
but then so are the use of non-supported 3rd party utilities on domain
controllers.