I would imagine buteman has probably hit the nail on the head. As you may know to add programs and delete programs you need to be the administrator and that may be the reason the other users might be blocked. The unblocking which you said you have ticked might just be so they can run the anti virus but because they are not administrators windows will not allow then to download the updates as that would count as adding programs.
Don't know if that is the case but it sounds reasonable.
This thread is now locked and can not be replied to.