.NET [VB.NET] Login system , little problem [VB.NET]

Code
#Region "Function"

    Function GetCPUID()

        Dim cpuInfo As String = String.Empty

        Dim mc As New ManagementClass("win32_processor")

        Dim moc As ManagementObjectCollection = mc.GetInstances()

        For Each mo As ManagementObject In moc

            cpuInfo = mo.Properties("processorID").Value.ToString()

        Next

        Return cpuInfo

    End Function

    Function SystemSerialNumber() As String

        Dim wmi As Object = GetObject("WinMgmts:")

        Dim serial_numbers As String = ""

        Dim mother_boards As Object = wmi.InstancesOf("Win32_BaseBoard")

        For Each board As Object In mother_boards

            serial_numbers &= ", " & board.SerialNumber

        Next board

        If serial_numbers.Length > 0 Then serial_numbers = _

            serial_numbers.Substring(2)

        Return serial_numbers

    End Function

    Function GetID()



        Dim Str1 As String = GetCPUID()

        Dim Str2 As String = SystemSerialNumber()

        Dim HWID As String = String.Empty

        Do

            If Str1 <> String.Empty Then

                HWID += Str1.Last

                Str1 = Str1.Remove(Str1.Count - 1)

            End If

            If Str2 <> String.Empty Then

                HWID += Str2.Last

                Str2 = Str2.Remove(Str2.Count - 1)

            End If

        Loop Until Str2.Count = 0 And Str2.Count = 0

        Return HWID

    End Function

#End Region