UAC came out over 10 years ago! In any case, NT is structured quite differently than the typical UNIX variants. When you log in, NT creates two access tokens for you. One with all your privileges, and one with admin rights masked out. When a process is launched it uses the non-admin one by default, even if you are the admin user.

When a thread requires admin privileges, NT will first check if your unmasked token already has those rights, in which case, it will prompt you (UAC) for permission to use the token. This is unlike traditional UNIX where the 'effective user' changes to root on a global level, not just per-thread. So you get 10 minutes (or w/e) as root to do your thing.