Purpose of the UAC box in Windows Vista?

UAC is the User Account Control. That is an added security measure to prevent certain manipulations to the system. That would include messing with the system root or the program files, adding or removing programs, installing drivers, installing ActiveX controls, adding or removing user accounts, configuring Windows Update, running programs with Administrator privileges, changing another users files, defragging the drive, etc. It is a way to temporarily grant Administrator privileges without having those privileges available all the time. That way, malware cannot do those things.