I seem to remember reading somewhere that the windows NT permission model at least is better designed than the Unix one. It allowed extremely granular tuning of permissions so that programs can be designed to target specifically just the permissions they need.
Of course, that was part of the problem, because it was much much easier just to assume everyone was an admin, meaning developers just asked for admin permissions, and users had to run as admin. It's not just the design of the system which can be faulted, but the laziness of the developers.
Of course, allowing lazy developers to write apps is one of the reasons that windows is so popular, ah well it would appear there's no easy answer - who knew?