A: The Power Users group is really a group that potentially has administrator level privileges, and those privileges were often abused. See the Microsoft article "Windows Confidential: Power to the Power User" and Mark Russinovich's "The Power in Power Users" article for great information on how Power User privileges can be abused.

Because of this abuse, in Windows XP the Power Users group was hidden from the UI to dissuade people from its use. And later, the decision was made in Windows Vista to actually strip the group of its rights so members are just typical users.