Windows Server 2008 Password Complexity Requirements

• Passwords cannot contain the user’s account name or parts of the user’s full name that exceed two consecutive characters.
• Passwords must be at least six characters in length.
• Passwords must contain characters from three of the following four categories:
• English uppercase characters (A through Z).
• English lowercase characters (a through z).
• Base 10 digits (0 through 9).
• Non-alphabetic characters (for example, !, $, #, %).

http://thebackroomtech.com/2008/03/10/windows-server-2008-password-complexity-requirements/