ASP.NET MVC disable Windows Identity

i use to FormsAuthenticationTicket (with form) for authorize users with [Authorize] attribute, when on IIS was allow Anonymous and Form Auths all working so good, BUT now need to switch to Windows and Form - for disable whole pages for Anonymous access. But when i try login by Windows then i have an access like a user because i use to User.Identity.IsAuthenticated for check is user login or not. How to disable Windows authority in this case.

web.config

<authentication mode="Forms">
  <forms name="Auth" loginUrl="~/Account/Login" defaultUrl="~/" timeout="30"/>
</authentication> 

i think about overwrite Authorize attribute but it not help with User.Identity.IsAuthenticated. ThankX

UPD: The same problem with User.Identity.Name etc...

UPD2: I think about Custom Attribute with some like this property:

public class LoggedAttribute : AuthorizeAttribute
{
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        return (base.AuthorizeCore(httpContext) && !IsWindows());
    }

    public static bool? _isWindows = null;

    public static bool IsWindows()
    {
        if (!_isWindowsAuth.HasValue)
        {
            if ((HttpContext.Current != null) && HttpContext.Current.User.Identity.IsAuthenticated)
            {
                _isWindows = new bool?(HttpContext.Current.User is WindowsPrincipal);
            }
            else
            {
                try
                {
                    AuthenticationSection section = (AuthenticationSection)WebConfigurationManager.OpenWebConfiguration(VirtualPathUtility.ToAbsolute("~")).GetSection("system.web/authentication");
                    _isWindows = new bool?(section.Mode == AuthenticationMode.Windows);
                }
                catch
                {
                    _isWindows = false;
                }
            }
        }
        return _isWindows.Value;
    }

}