Microsof has been fined £485m by the European Commission for a botched software update that meant millions of customers were never presented with its browser ballot screen.
The EC had insisted Microsoft introduce the ballot over antitrust concerns over its practice of bundling its own browser in with its operating system, to the detriment of rivals.
Last year, the EC confirmed its intention to charge Microsoft over the removal of the browser ballot when it updated its software.
The software giant faced the threat of a fine as high as $7.4bn so in some respects Microsoft has got off lightly.
"Legally binding commitments reached in antitrust decisions play a very important role in our enforcement policy because they allow for rapid solutions to competition problems," said commission vice president in charge of competition policy, Joaquín Almunia.
"A failure to comply is a very serious infringement that must be sanctioned accordingly."
The incidents arose after an investigation by the EC found that Microsoft had failed in its legal obligation to provide a screen that presents users of its Windows operating system with alternative browser such as Opera or Firefox when they first attempt to go online (see video below).
However, a Windows 7 Service Pack 1 update rolled out in February 2011 resulted in the removal of this system, meaning millions of users were not presented with the screen.
Compounding this, Microsoft claimed in its annual compliance report to the EC in December 2011 that it was fulfilling its obligations under the 2009 ruling.
Microsoft acknowledged the issue but didn't comment directly on the fine.
"We take full responsibility for the technical error that caused this problem and have apologised for it," it said.
"We provided the Commission with a complete and candid assessment of the situation, and we have taken steps to strengthen our software development and other processes to help avoid this mistake - or anything similar - in the future."
A video below shows how the browser ballot choice systems works:
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