Windows NT has had users and permissions providing similar functionality to what Unix offers for a very long time[0]. NT ACLs have actually been more sophisticated than standard Unix permissions for at least ten years.
The problem on Windows is the defaults and the behavior of third-party developers. Windows developers worked for a long time with the expectation that the system was single-user (Win9x) or that the user had administrator privileges, making doing anything else annoying at best. XP defaulted to making everybody an administrator, at least in the home edition. That has changed with Vista and Win7.
[0] I think some versions targeted at home users do not expose the full ACL functionality.