C# - Grey out window

When you launch the login form from the main form, you should set its Owner property to 'this'.

The login form will then be owned by the main form and will always appear on top of it:

LoginForm login = new LoginForm();
login.Owner = this;
login.Show();
this.IsEnabled = false; // disable main form

You then need to handle the login form's Closed event to re-enable the main form:

private void Window_Closed(object sender, EventArgs e)
{
   this.Owner.IsEnabled = true;
   this.Owner.Activate();
}