To counterpoint that... I've had to clean up 3 laptops in the past fortnight with junk installed that UAC *would* have prevented. In each and every case UAC had been disabled because they'd read on an IT forum that it just "gets in the way"
It's there as an extra layer. It can't save someone from their own stupidity. Let me give you a "for example".
Jimmy is installing an application with UAC active. He gets prompted and because he knows that he needs to make system level changes in order to install this application he accepts that the change is authorised. The application installs, and Jimmy continues about his business.
Billy is surfing the internets. He only ever uses trusted sites, because he read that trawling for pr0n and warez could land him in trouble, especially as he's at work. He was told by Dave in Accounts that UAC is nothing more than an annoyance, so he turned it off. He visits
www.hexus.net and immediately doesn't receive a UAC prompt. What he doesn't know is that the adservers have been compromised, and one of the banners has silently installed malware on his machine. As he's not aware of it, he continues about his business only to find that a few days later his bank has been cleared out/his machine is part of a massive spam-spewing botnet/make up your own scenario.
Now I'm not saying that Hexus' adservers have been compromised, I'm using it as an example of a trusted site that serves ad banners from a third party server (at least I'm guessing that intellitxt.com is a 3rd party - it could well be owned by DR, but it serves the purpose as an example) over which they have little to no control. A UAC prompt, 5 seconds of thought is all it takes to decide if a change is authorised. If you'd prefer not to be made aware that a change is taking place and given the opportunity to stop it that's your choice, my (educated) choice is to keep UAC enabled.