|At the RSA 2008 confab in San Francisco, Microsoft admitted that UAC was designed, in fact, to annoy. Microsoft's David Cross came out and said so: "The reason we put UAC into the platform was to annoy users. I'm serious," said Cross.
UAC's real purpose is quite simple: it's meant to trip whenever a routine attempts to elevate security privileges, and get in your face. As we have reported before, this has two goals: a) it give users a chance to approve of the elevation in the off chance that something wrong is happening, and b) it encourages developers to design their software such that privilege elevations aren't needed in the first place. The latter is really the point of UAC, since users have absolutely zero control over the privilege requests their applications make (other than to chose not to install said apps).
Seems about right.