Not an expert on windows security, but the traditional answer is the fundamental user handling in Unix solves a lot of problems. I admit ignorance to the validity of the claim, a detailed comparison would be appreciated.
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.
Why is the first account you create an admin account by default. If they wanted the use-case that you imply is supported, they would have you create an administrator account and then would ask you to create a regular user to use day-to-day.
Or they would more fully embrace the su/sudo routine that linux uses.
Edit: Also, UAC is basically sudo.
UAC is defective from the standpoint that every school and corporation I've been at disable it, and those that done click Accept every time without reading or considering what they're clicking on. I understand this is partly a user problem, but damn, I'll do one tiny thing before remembering to disable it and I have to Accept 30 things before I'm done.