Microsoft Corp.'s Steve Ballmer tried new operating systems, new gadgets and new management structures. But the times caught up with a man who helped build one of the greatest companies of the 20th century.

Mr. Ballmer's surprise retirement announcement Friday follows years of criticism about the waning growth and stagnant stock price of Microsoft, a force in the personal-computer era whose power was once so great that U.S. regulators sought to break up the company.

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