Info:
The annoyance of UAC pop-ups is unfortunately front-loaded: they happen most often when Vista is new and you're installing new applications and downloading files more often. The temptation is strong to kill them, because it's easy. Just go to Start > Control Panel > Security > Security Center > Other Security Settings. But the problem with shutting off UAC is that other security-conscious Vista components may decide it's just not safe to run without them, like the Windows Installer.

Using the following switch rather than disabling UAC is a good thing because it leaves Vista security in place.

Instructions for turning off the nag screen:
Click Start and Control Panel. Switch to Classic View (if you haven't already) and click on the Administration Tools icon. In the list that opens click on Local Security Policy, and in the next window, Local Policies (a tiny bit redundant, but all UIs can't be perfect -- If UAC is running you'll get a UAC pop-up somewhere in here). In the Local Policies list click Security Options, and scroll down to "User Account Control: Behavior" (the full title of the policy is "User Account Control: Behavior of the elevation prompt for administrators in Admin Approval Mode" but the window barely opens that far). Double-click the title and in the dialog box change its setting from "Prompt for Consent" to "Elevate without prompting." Click OK and the urge to tear your hair and scream at your PC will be greatly diminished in the future.

Full Article

Or use Tweak UAC program for an easy switch for on and off or silent.

Tweak UAC Website