See the MS KB q225230
To implement strong password requirements for your domain, configure a group policy object linked to your domain:
Start the Group Policy Editor Microsoft Management Console (MMC) snap-in, focused on the appropriate grouppolicy object. This must be a group policy object linked to the entire domain. You can directly edit the Default Domain Controllers Policy GPO if you are using this policy object.
A quick method of starting the Group Policy Editor MMC snap-in with its focus pointed to this GPO is:
Right-click the domain object in the Active Directory Users and Computers MMC snap-in, and then click Properties.
Click the Group Policy tab.
Click the Default Domain Policy GPO link, and then clickEdit.
Navigate to the following node in the group policy object:
Group Policy Object Policy\Computer Configuration\Windows Settings\Security Settings\Account Policies\Password Policy
Double-click Passwords must meet complexity requirements of installed password filter to produce the application dialog box.
Change the template setting to Enabled to activate the strong password requirement.
As with all policy settings, the change is not applies until the next time group policy objects are applied to your domain controllers.
regards,
Leopold.