enable Site Security resulted in error

Greeting,         I am running GE 2.1 on Win2003 X64. When I enabled site security with default XMLMembershipProvider and Default XmlRoleProvider, I got: ~~~~~~~~ Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.   from the login panel. I have .NET 3.5sp1, any idea what might be missing?   Thank you,   Yeong (ylin@attain.com)