http://support.microsoft.com/kb/825069

By default, the rights and permissions that are granted to the Power Users group include those rights and permissions that are required to allow members of the Power Users group to modify computer-wide settings, to install drivers, and to run (or install) non-certified programs. For example, a member of the Power Users group could install a malicious program or a DLL, and then cause the administrator or a system service to run the malicious program or the DLL. By using this technique or other techniques, the member of the Power Users group may be able to gain additional rights and permissions on your computer, including complete administrative credentials.
How would one go about accomplishing this?