How to do Windows Authentication (Users in the Intranet) in

You can check the user against active directory and log them in immediately without the login page showing (or just deny them access).  These methods should help you to figure out if the user is valid.

After you have their information, you can authenticate them straight away with this:

FormsAuthentication.RedirectFromLoginPage(currentUser, false);

        // Get the currently logged in person's user name. (with the domain name)
        public static string GetDomainUserName()
        {
            return System.Web.HttpContext.Current.Request.LogonUserIdentity.Name;
        }

        // Get just the currently logged in person's user name (without the domain name)
        public static string GetShortUserName()
        {
            string rtnVal = System.Web.HttpContext.Current.Request.LogonUserIdentity.Name;

            string[] parts = rtnVal.Split('\\');
            rtnVal = parts[parts.Length - 1]; // return only the last part - that's the user name

            return rtnVal;
        }

        // Get just the currently logged in person's domain name (without the user name)
        public static string GetDomainName()
        {
            string rtnVal = System.Web.HttpContext.Current.Request.LogonUserIdentity.Name;

            string[] parts = rtnVal.Split('\\');
            rtnVal = parts[0]; // return only the last part - that's the user name

            return rtnVal;
        }

///

        /// Determine if user Id and password represent a valid Active Directory User
        /// This method assumes the execution is within the same domain as the tested
        /// user - not sure if this can always be relied upon so the next function
        /// performs the same test but allow passing in the domain name as an argument
        ///

        ///
        ///
        ///
        public static bool ValidateActiveDirUser(string userId, string password)
        {
            PrincipalContext adContext = new PrincipalContext(ContextType.Domain);

            using (adContext)
                return adContext.ValidateCredentials(userId, password);
        }

        ///

        /// Validate a user is a valid AD member in a specified domain
        ///

        /// Provide the user's user name (ID)
        /// Provide the user's password (plain text)
        /// Provide the user's domain name
        ///
        public static bool ValidateActiveDirDomainUser(string userId, string password, string domainName)
        {
            PrincipalContext prncContext = new PrincipalContext(ContextType.Domain, domainName);

            using (prncContext)
                return prncContext.ValidateCredentials(userId, password);
        }