The Windows 7 service pack 1, rolled out between mid-2011 and mid-2012, failed to offer consumers a choice of browser, leading to the EU investigation that resulted in today's fine.
Microsoft said the failure was the result of a technical error and that procedures have since been tightened.
In what was seen as an acknowledgement of the severity of the mistake, the board cut the bonus of chief executive Steve Ballmer last year, according to company's annual reports.
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The Prime Minister will join hundreds of military personnel in Liverpool on Saturday as the country marks Armed Forces Day.
The Labour MP expects the death toll in the Grenfell fire to be well above 79 and blames the disaster on Tory attitudes to social housing.
At least 27 high-rise blocks of flats in 15 local authority areas have failed fire cladding safety tests, the Government has announced.