The European Union has fined software giant €561 million for failing to comply with a 2009 anti-competition ruling on web browser software.
In 2007, Norwegian browser maker Opera filed an antitrust to the European Commission, calling on Microsoft to gives customers a choice of browser when they installed its Windows operating system.
In 2009, the EC upheld Opera's complaint and accused Microsoft of "illegally tying Internet Explorer to Windows". It order the company to give users a choice of browser on installation.
Microsoft agreed to present users with a Browser Screen Choice (BSC) pop-up window during installation, given them the choice Apple's Safari, Google's Chrome, Opera and Mozilla's Firefox. At the time, Opera Software CEO Jon von Tetzchner called the decision a "victory for the future of the Web".
However, Microsoft failed to show the BSC to users who installed Windows 7 Service Pack 1 between February 2011, when the pack was first released, and July 2012.
In July, the EU began investigating whether Microsoft was complying with the 2009 ruling – the first time it has launched such a probe.
Microsoft apologised that month, blaming the omission on a "technical error".
"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," EU competition commissioner Joaquin Almunia said in a statement.
"Of course, such decisions require strict compliance," he said. "A failure to comply is a very serious infringement that must be sanctioned accordingly."
It is not the first time Microsoft has faced action from the EC for tying its software to its operating systems.
In March 2004, the EC ruled that the company had harmed competition by including Windows Media Player with its operating system and ordered the company to sell a version without it.