IF you want to create a single sign on application, you don't need to be showing a logon form - that defeats the purpose of SSO!
Instead, you should be using the WindowsPrincipal
] and WindowsIdentity
] objects to retrieve the details of the current logged on user.
WindowsPrincipal principal = new WindowsPrincipal(WindowsIdentity.GetCurrent());
Thread.CurrentPrincipal = principal;
You can then enable application areas based on the users group permissions