Software giant could face a barrage of class-action suits.

Microsoft has collected $10 billion in overcharges to consumers over the past three years thanks to its monopoly on the PC operating system, consumer groups alleged in a study released Friday. As a result, officials of the Consumer Federation of America and other consumer groups said, the company could face a barrage of class-action suits designed to recoup those supposed overcharges if courts find Microsoft liable in the government antitrust suit under way here. The report was prepared by the CFA, the Media Access Project and the U.S. Public Interest Research Group. "We think those overcharges should be returned," said Mark Cooper, research director of the CFA, who estimates the overcharges amount to some $35 to $45 per computer.