Permissions and rights have been removed from the Power Users group in Windows Vista
In Windows Vista and later Windows operating systems, the permissions and rights that are associated with the Power Users group have been removed. Therefore, the Power Users group no longer has elevated permissions. Previously, the permissions that were granted to members of the Power Users group permitted them to effectively take full administrative control of the operating system. A decision was made in Windows Vista to remove these elevated permissions in order to promote more secure deployments of the operating system.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB2028493.