http://support.microsoft.com/kb/232714/
Enable Auditing of Active Directory
// To enable auditing of Active Directory:
- Start the Active Directory Users and Computers snap-in by clicking Start, pointing to Programs, and then pointing to Administrative Tools.
- On the View menu, click Advanced Features.
- Right-click the Domain Controllers container, and then click Properties.
- Click the Group Policy tab.
- Click Default Domain Controller Policy, and then click Edit.
- Double-click the following items to open them: Computer Configuration, Windows Settings, Security Settings, Local Policies, Audit Policy.
- In the right pane, open Audit Directory Services Access.
- Click the appropriate option(s): Audit Successful Attempts and/or Audit Failed Attempts.
- Open the Security Log to view logged events.
http://technet.microsoft.com/en-us/library/cc739112(WS.10).aspx
Refresh GPO.









