Subroutine MapDirectory uses My.Computer.FileSystem.GetFiles to list the files in a directory. It then uses My.Computer.FileSystem.GetDirectories to list the directories in that one and recursively calls itself to search them.

' Search the directory subtree and make a list of the files.
' (In a real application, you would probably do something
' with the files other than putting them in a Listbox.)
Sub MapDirectory(ByVal dir As String, ByVal lst As ListBox)
    With My.Computer.FileSystem
        ' List this directory's files.
        For Each file As String In .GetFiles(dir)
            'TODO: Do something with the file.
            lstFiles.Items.Add(file)
        Next file

        ' Search child directories.
        For Each folder As Object In .GetDirectories(dir)
            'TODO: Do something with the folder.
            MapDirectory(folder, lst)
        Next folder
    End With
End Sub