NTREGOPT is a free registry compactor that improves system performance by compacting the registry
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.
NTREGOPT should be used regularly, but especially after a install/uninstall process so that it minimizes the size of the registry and optimizes it's acces.
NTREGOPT 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 physically defrag the registry files on the drive (as the PageDefrag program from SysInternals does).
The optimization done by NTREGOPT is simply compacting the registry hives to the minimum size possible.