w/c.
sorry for late Repl.
Due to security permissions, windows does not allow to validate Password. You need to find some other-way.
http://support.microsoft.com/kb/316748 this link will give you a brief idea abt security etc.
Most of the time these are the Intranet Web Applications that uses AD Uses Authe / Machine Users.
These URLs (Intranet) are placed in Trused Site zone and verified against Username only.
still if u need password validation, Search on ASP.Net forum or Expert Exchange.
Let me know if u want something more.