Windows installation distributive type.

enum _PRL_GUEST_DISTRO_VERSION_ENUM {
  PGD_UNKNOWN = 0,
  PGD_WINDOWS_VISTA = 1,
  PGD_WINDOWS_XP = 2,
  PGD_WINDOWS_XP_32 = PGD_WINDOWS_XP,
  PGD_WINDOWS_XP_64 = 3,
  PGD_WINDOWS_VISTA_32 = PGD_WINDOWS_VISTA,
  PGD_WINDOWS_VISTA_64 = 4
};