Power Users are really somewhat like Administrators under a cloak or
Administrators in waiting, to quote one security expert: "Power Users
are simply Administrators who have not made themselves Administrators
yet."
Power Users have broad powers and with their power to install and run
any software they can easily gain all administrator privileges. When
granting users Power User rights you should really ask yourself if you
can trust them as administrators, if the answer is no then they should
probably not be trusted as Power Users.
http://support.microsoft.com/Default.aspx?kbid=825069
A member of the Power Users group may be able to gain administrator
rights and permissions in Windows Server 2003, Windows 2000, or Windows XP
http://blogs.technet.com/markrussinovich/archive/2006/05/01/the-power-in-power-users.aspx
Mark's Blog : The Power in Power Users
John