The most basic element and direct benefit of UAC's technology is simply making Windows more standard-user friendly. The showcase example is the difference between the privilege requirements of setting the time zone on Windows XP and Windows Vista. On Windows XP, changing the time zone—actually, even looking at the time zone with the time/date control panel applet—requires administrative rights. That's because Windows XP doesn't differentiate between changing the time, which is a security-sensitive system operation, from changing the time zone, which merely affects the way that time is displayed.
Full Article: TechNet