User Account Control (UAC) is a technology and security infrastructure introduced with Windows Vista. It aims to improve the security of Microsoft Windows by limiting application software to standard user privileges until an administrator authorizes an increase in privilege level.
Many applications developed prior to Windows Vista are not working correctly with UAC or requires special measures during installation and setup. Also, many users find UAC annoying. So many prefer to disable UAC.
A system reboot is required to apply changes.