Microsoft's old permissions were VERY lax. AS in low permission user could do and change things as if they were the administrator. A lot of programs expected this behavior, even though they really should not have. But hey it worked. Microsoft has been tithing it up for years. Even 98 to XP and some of the service packs.
That an a newer diver model that avoid bad drivers from taking out the whole system.
Now if they could fix stuff faster then they break it, they may have a an OS there.