NTREGOPT

Similar to Windows 9x/Me, the registry files in an NT-based system can become fragmented over time, occupying more space on your hard disk than necessary and decreasing overall performance. You should use the NTREGOPT utility regularly, but especially after installing or uninstalling a program, to minimize the size of the registry files and optimize registry access.
The program works by recreating each registry hive "from scratch",thus removing any slack space that may be left from previously modified or deleted keys.
Note that the program does NOT change the contents of the registry in any way, nor does it defrag the registry files "on-disk" (as the PageDefrag program from SysInternals does). The optimization done by NTREGOPT is simply compacting the registry hives to the minimum size possible.
System Requirements-- Windows NT/2000/XP

REGSEEKER

RegSeeker is the perfect companion for your Windows registry ! RegSeeker includes a powerful registry cleaner and can display various informations like your startup entries, several histories (even index.dat files), installed applications and much more ! With RegSeeker you can search for any item inside your registry, export/delete the results, open them in the registry. RegSeeker also includes a tweaks panel to optimize your OS ! RegSeeker is FREE for PERSONAL USE ONLY ! This is a safe registry program. I have not found it deleting any important registry entry and thereby cause a system crash or any other installed program crash.

REGCLEAN

This is a registry cleaner utility for windows from Microsoft. Not many are aware of this registry cleaner from Microsoft, as most use the private softwares available for registry cleaning. You can get the program from microsoft site or on a search on the web from any private site. This cleaner has been shipped with the MS Cd of windows 95/98 and thereafter under "tools" folder.