In XP, you could always change permission without logging off, by using the runas command or Run as... in the context menu. I think you could also set shortcuts to run programs as Administrator.
So you could run as a non privileged users, and you'd only need to log in as Administrator for some very specific tasks.
However, there were many applications that didn't play nice with that model and required you to run as Administrator. By changing the default, MS had to do a big push to make software makers abide by the rules (which had been in place since XP (edit: in the mainstream branch) but were often ignored)
You could do that under XP, but needed a separate user account for administrator. It was also more of a hassle, as you always had to enter the password when starting a program as Administrator. I ran with such a setup on my desktop, and remember that to install programs I usually logged out and logged in as Administrator again.
Vista introduced a model that was almost as good, but made it more user-friendly and the default.
So you could run as a non privileged users, and you'd only need to log in as Administrator for some very specific tasks.
However, there were many applications that didn't play nice with that model and required you to run as Administrator. By changing the default, MS had to do a big push to make software makers abide by the rules (which had been in place since XP (edit: in the mainstream branch) but were often ignored)