"For defence in depth, developers may wish to use the Microsoft Anti-Cross Site Scripting Library to encode output. This library differs from most encoding libraries in that it uses the "principle of inclusions" technique to provide protection against XSS attacks. This approach works by first defining a valid or allowable set of characters, and encodes anything outside this set (invalid characters or potential attacks). The principle of inclusions approach provides a high degree of protection against XSS attacks and is suitable for Web applications with high security requirements."

Article Link: http://blogs.msdn.com/ace_team/archive/2006/11/20/microsoft-anti-cross-site-scripting-library-v1-5-is-done.aspx