Given the length of time the latest violation went on, and the number of users affected, "it does strain credibility'' that Microsoft wouldn't have known it had failed to keep its part of the agreement, Sabino added.
For its part, Microsoft was apologetic.
"We take full responsibility for the technical error that caused this problem and have apologized for it,'' the company said in a statement. "We provided the Commission with a complete and candid assessment of the situation, and we have taken steps ... to help avoid this mistake -- or anything similar -- in the future.''
Microsoft's Internet Explorer still has a 56 percent market share for Internet browsers on personal computers, according to statistics by Net Applications. Mozilla's Firefox has 20 percent while Google's Chrome has a 17 percent share. Microsoft is required to continue to offer consumers a choice of browsers through 2014.
But the wider competitive landscape has changed greatly since the Commission started its action against Microsoft. Software applications on mobile phones usually bypass browsers entirely. Tech companies are now often less concerned with Internet browsers and more concerned about Google Inc.'s dominance in Internet search technology, Facebook's dominance in social networking, and Apple's dominance on mobile devices.
"For technology companies, often the best policeman is technological progress itself,'' Sabino said.
Rather than imposing unilateral fines, Almunia has advocated negotiated settlements since he took over as commissioner in 2010. He believes that competition issues are best resolved quickly and that slapping big fines on companies years after the fact does little to help consumers.
But he said the whole point of a settlement is undermined when companies then don't abide by its terms.
"They must do what they committed to do, or face the consequences,'' he said.
Almunia conceded that the Commission had been "naive'' in appointing Microsoft itself to oversee compliance with the agreement, and said the Commission won't allow that in the future.