def GetMainIsoUrl(pro):
  """Gets the main .iso URL.

  If |pro| is False, downloads the Express edition.
  """
  prefix = 'http://download.microsoft.com/download/'
  if pro:
    return (prefix +
        'A/F/1/AF128362-A6A8-4DB3-A39A-C348086472CC/VS2013_RTM_PRO_ENU.iso')
  else:
    return (prefix +
        '7/2/E/72E0F986-D247-4289-B9DC-C4FB07374894/VS2013_RTM_DskExp_ENU.iso')