Tip 2635 ยป How do I defragment the Active Directory database states:

During the Garbage Collection process which occurs every 12 hours, online defragmentation optimizes data storage and reclaims space for new objects, but it does not reduce the size of the Active Directory database (Ntds.dit).

To cause the Garbage Collection process to log a message in the Directory Service event log that states how much disk space might be freed up by performing an offline defragmentation:

1. Open a CMD.EXE window.

2. Using REG.EXE, built into Windows XP, Windows Server 2003, and later operating systems, or installed from the Windows 2000 Support Tools, type the following command and press Enter:

REG ADD HKLM\SYSTEM\CurrentControlSet\Services\NTDS\Diagnostics /V "6 Garbage Collection" /T REG_DWORD /F /D 1