User Account Control (UAC) in Windows Vista is one of the most prominent security features in the operating system. It is recommended by Microsoft that this be left on for your users but in some cases this might not be appropriate for some users. For instance, this will often be turned off on workstations belonging to IT staff (or administrator who are fed up with the constant warnings). If you do need to disable this for a particular group of computers you can do this with Group Policy.
The UAC setting can be found under Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\Security Options. If you set this to disabled and apply it to an OU containing the computers you do not want to use UAC for then this will effectively accomplish your goal.