Microsoft has been fined 561 million euros ($731m; £484m) for failing to promote a range of web browsers, rather than just Internet Explorer program, to users in the European Union (EU).
It introduced a Browser Choice Screen pop-up in March 2010 as part of a settlement following an earlier EU competition investigation.
But the US company dropped the feature in a Windows 7 update in February 2011.
Microsoft said the omission had been the result of a "technical error".
But competition commissioner Joaquin Almunia said the action was unprecedented, adding he wanted to deter any company from the "temptation" of reneging on such a promise.
In theory the watchdog could have fined the firm 10% of its global annual revenue, which would have totalled $7.4bn based on its 2012 report.
"The European Commission is sending a firm signal in this first case of its type that it will not tolerate failure by a company to comply with the commitments it gave to settle an antitrust infringement procedure," said Tony Woodgate, a competition lawyer at Simmons & Simmons.