At Windows Vista lab in Redmond before the release of Beta 2, Microsoft developers showed off the new OS to a room full of MVPs and enthusiasts. But even the company’s most loyal fan base turned ugly when User Account Control took the stage. Now, Microsoft is begging users not to disable the controversial feature. User Account Control, or UAC, is a fundamental security change coming in Windows Vista and one of the most important additions to protect users from threats, Microsoft says. But the company is struggling to find a balance between security and usability.