Returns the service key.

def _GetServiceKey():
  """Returns the service key."""
  global _service_key

  if _service_key is None:
    hive = getattr(winreg, config.CONFIG["Client.config_hive"])
    path = config.CONFIG["Client.config_key"]

    # Don't use winreg.KEY_WOW64_64KEY since it breaks on Windows 2000
    _service_key = winreg.CreateKeyEx(hive, path, 0, winreg.KEY_ALL_ACCESS)

  return _service_key