It sounds like there is some program on your computer that is resetting
the setting.
Windows does not do this automatically, although it does display a
bubble popup telling you that UAC is disabled, and if you click on the
bubble, it will turn UAC back on.
Do you have any security software installed that monitors system
settings for changes? You might check into those programs and see if
they are changing the setting back.
--
-JB
Microsoft MVP - Windows Shell/User
Windows Vista Support FAQ -
http://www.jimmah.com/vista/