Password filtering comes in a DLL that you add to your system. To use it, copy it to your Primary Domain Controller (PDC) and to all your Backup Domain Controllers (BDCs) in case the PDC fails or roles change. Make sure that your %SYSTEMROOT%\System32 directory contains the DLL; then adjust one Registry entry on each domain controller to enable it. Run regedit32, and then follow these steps:
1. Locate HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
2. Add the following:
Value Name: Notification Packages
Data Type: REG_MULTI_SZ
3. Click OK, and then quit the Registry editor.
4. Shut down and restart NT.
You'll find the source code for this DLL in Microsoft's Knowledge Base article Q151082 on the TechNet CD-ROM or on the Web, http://www.microsoft.com/kb.