BERLIN  European regulators dropped their antitrust case against Microsoft on Wednesday after the software maker agreed to offer consumers a choice of rival Web browsers. The move ended a decade of legal strife that cost the world's top maker of software 1.67 billion euros in fines and penalties and forced it to alter the way it did business in Europe.

The agreement, announced in Brussels by the European competition commissioner, Neelie Kroes, calls for Microsoft to give users of its Windows operating system a choice of 11 browsers that compete with its Internet Explorer, made by companies like Apple, Google and Mozilla.

The five-year deal is an unprecedented concession for a company that, since its founding by Bill Gates and his partners in 1975, had largely defined, exploited and defended the advantages and rights of proprietary, commercial software.

When Microsoft first entered the cross hairs of European regulators in 1998 for including its media player with Windows, which operates 90 percent of the computers around the world, and for its use of confidential coding to favor its desktop and server software, the company fought back with legal guns blazing.