Private Sub timUhr_Tick(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles timUhr.Tick
 
        Dim Pad As Char = "0"
 
        If Action = "Up" Then
            TSpan = DateTime.Now.Subtract(DiffZeit)
            AnzeigeMilli = TSpan.Milliseconds
            AnzeigeSec = TSpan.Seconds
            AnzeigeMin = TSpan.Minutes
            txtZeit.Text = AnzeigeMin.PadLeft(2, Pad) & ":" & _
              AnzeigeSec.PadLeft(2, Pad) & ":" & CStr(AnzeigeMilli).PadLeft(3, _
              Pad)
        End If
 
        If Action = "Countdown" Then
            If Me.txtZeit.Text <> "00:00:000" Then
                txtZeit.Text = CStr(i.Subtract(Now).Minutes).PadLeft(2, Pad) & _
                  ":" & CStr(i.Subtract(Now).Seconds).PadLeft(2, Pad) & ":" & _
                  CStr(i.Subtract(Now).Milliseconds).PadLeft(3, Pad)
            Else
                Dim nRet As Integer
                nRet = PlayWav(Application.StartupPath & "/whistle.wav", _
                  SND_ASYNC)
                Me.timUhr.Enabled = False
            End If
        End If
 
 
    End Sub

Always change a running system...or you will never learn!