Problem with LDAP authentication in asp.net application

Public Class LDAP
    Public Const SECURITY_IMPERSONATION_LEVEL_SecurityAnonymous As Integer = 0
    Public Const SECURITY_IMPERSONATION_LEVEL_SecurityIdentification As Integer = 1
    Public Const SECURITY_IMPERSONATION_LEVEL_SecurityImpersonation As Integer = 2
    Public Const SECURITY_IMPERSONATION_LEVEL_SecurityDelegation As Integer = 3

    Public Const LOGON32_PROVIDER_DEFAULT As Integer = 0
    Public Const LOGON32_PROVIDER_WINNT35 As Integer = 1
    Public Const LOGON32_PROVIDER_WINNT40 As Integer = 2
    Public Const LOGON32_PROVIDER_WINNT50 As Integer = 3

    Public Const LOGON32_LOGON_INTERACTIVE As Integer = 2
    Public Const LOGON32_LOGON_NETWORK As Integer = 3
    Public Const LOGON32_LOGON_BATCH As Integer = 4
    Public Const LOGON32_LOGON_SERVICE As Integer = 5
    Public Const LOGON32_LOGON_UNLOCK As Integer = 7
    Public Const LOGON32_LOGON_NETWORK_CLEARTEXT As Integer = 8
    Public Const LOGON32_LOGON_NEW_CREDENTIALS As Integer = 9

    Public Const ERROR_LOGON_FAILURE As Integer = 1326

    <DllImport("advapi32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
    Public Shared Function LogonUser(ByVal lpszUsername As String, ByVal lpszDomain As String, ByVal lpszPassword As String, ByVal dwLogonType As Integer, ByVal dwLogonProvider As Integer, ByRef phToken As IntPtr) As Boolean
    End Function

    <DllImport("advapi32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
    Public Shared Function RevertToSelf() As Boolean
    End Function

    <DllImport("kernel32.dll", CharSet:=CharSet.Auto)> _
    Public Shared Function CloseHandle(ByVal handle As IntPtr) As Boolean
    End Function

    <DllImport("advapi32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
    Public Shared Function DuplicateToken(ByVal hToken As IntPtr, ByVal impersonationLevel As Integer, ByRef hNewToken As IntPtr) As Integer
    End Function

    Public Shared Function Login(ByVal Username As String, ByVal Password As String) As Boolean
        Dim secPerm As New SecurityPermission(SecurityPermissionFlag.UnmanagedCode)
        secPerm.Assert()

        Dim user As WindowsIdentity = Nothing

        Dim refToken As IntPtr = IntPtr.Zero
        Dim loggedIn As Boolean

        loggedIn = LDAP.LogonUser(Username, "LDAP://LDAP connection string";, Password, LDAP.LOGON32_LOGON_NETWORK_CLEARTEXT, LDAP.LOGON32_PROVIDER_DEFAULT, refToken)

        'If loggedIn = True Then
        '    user = New WindowsIdentity(refToken, "NTLM", WindowsAccountType.Normal, True)
        'End If
        'CodeAccessPermission.RevertAssert()

        'Return user

        Return loggedIn


    End Function
End Class