This code will allow you to check to make sure the entered username and password is valid for what ever domain you specify. If you enter the local machine name as the domain it will also validate the windows account.

Code:
Public Class LoginForm
    Declare Function LogonUserA Lib "advapi32.dll" (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 Integer

    Private Enum LogonType As Long
        LOGON32_LOGON_INTERACTIVE = 2
        LOGON32_LOGON_NETWORK = 3
        LOGON32_PROVIDER_DEFAULT = 0
        LOGON32_PROVIDER_WINNT50 = 3
        LOGON32_PROVIDER_WINNT40 = 2
        LOGON32_PROVIDER_WINNT35 = 1
    End Enum

    Private Sub cmdOK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdOK.Click
        Dim Result As String

        Result = ValidateUser(txtUsername.Text, txtPassword.Text, txtDomain.Text)

        If Result = "True" Then
            MsgBox("User Validated")
        Else
            MsgBox(Result, MsgBoxStyle.Critical, "Error")
        End If
    End Sub

    Private Function ValidateUser(ByVal UserName As String, ByVal Password As String, ByVal Domain As String) As String
        Dim Token As IntPtr = IntPtr.Zero

        If LogonUserA(UserName, Domain, Password, LogonType.LOGON32_LOGON_INTERACTIVE, LogonType.LOGON32_PROVIDER_DEFAULT, Token) Then
            Return True
        Else
            Try
                Throw New System.ComponentModel.Win32Exception(System.Runtime.InteropServices.Marshal.GetLastWin32Error())
            Catch ex As Exception
                Return ex.Message
            End Try
        End If
    End Function
End Class