This is what happens:
1.Windows Vista has Defender integrated and is impossible to remove.
2.If with Vista you use Windows Live OneCare, Defender will always be
disabled or Stopped, because WLOC does its work. You can check this accessing
Services.
3.If you want to update Defender, first Start it in the Services and use the
About for update. Remember that if you use WLOC, it will disable Defender in
the next restart.
4.If you use XP without WLOC, you can install, remove, reinstall, and update
Defender as you wish.
5.If you use XP with Defender version 1.5 and above, and WLOC at the same
time, the Defender will be disabled by WLOC. So, that is why, if you use XP
and WLOC 1.5, remove Defender.
6.Remember that Defender version 1.5 is the one specifically designed for
Vista, and behaves the same in both XP and Vista.
This is all I know on the subject.
Efrain