The European Commission has fined Microsoft 561 million euros ($US731 million) for failing to stick to its promise to offer customers a choice of internet browsers.
It is one of the largest fines ever imposed by Brussels in such a case.
Microsoft introduced a Browser Choice Screen pop-up in March 2010 as part of a settlement following an earlier EU competition investigation. The company dropped the feature in a Windows 7 update in February 2011.
Microsoft said the omission was the result of a "technical error". The BBC reports it has reintroduced the browser choice facility.
Competition Commissioner Joaquin Almunia said the action was unprecedented, and he wants to deter any company from the "temptation" of reneging on such a promise.