FRAMINGHAM (11/07/2003) - The U.S. Federal Trade Commission has filed suit against a company that allegedly exploited a vulnerability in Microsoft Corp.'s Windows system to barrage users with pop-up ads using a feature intended for administrative alerts. The tactic involved the Windows Messenger Service, a software feature distinct from Microsoft's similarly named MSN Messenger and Windows Messenger instant-messaging applications. The Messenger Service is part of the Windows XP and Windows 2000 operating systems that let network administrators send messages to users, such as notifications about the status of print jobs. The feature proved problematic as external parties figured out how to hijack the Messenger Service and use it to send unsolicited information such as ads to Internet-connected computers. The company targeted by the FTC, D Squared Solutions LLC, used Messenger Service to flash ads touting its pop-up blocking software, which it sold for US$25 to $30, according to the FTC's complaint. D Squared officials could not be reached for comment.