Imports System.IO
Public Class Form1
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim ordner As String = "C:\Programme"
        Dim files As New List(Of String)(Directory.GetFiles(ordner, "*.txt")) ' 
        ' die txt-dateien im Ordner selbst
        Dim dlist As New List(Of String)(Directory.GetDirectories(ordner))    ' 
        ' die Unterordner des Ordners
        For Each dir As String In dlist                                         _
                    ' alle UO durchlaufen
            files.Add("Ordner: -----> " & dir)                               _
                         ' Name des UO
            files.AddRange(Directory.GetFiles(dir, "*.txt", _
            SearchOption.AllDirectories)) ' txt-Dateien des UO
        Next
        ListBox1.DataSource = files ' Ergebnis anschauen
    End Sub
End Class