The European Commission is to re-investigate Microsoft after it was accused of failing to give customers a choice of alternative web browsers.
A similar trial in 2009 ruled that Microsoft must show a ballot screen when a PC was first used to give customers the option to install alternative browsers, including Opera, Chrome and Firefox.
We take compliance with our decisions very seriously. And I trusted the company's reports were accurate.
This was introduced in February 2010, but it has been claimed that this has not always happened.
The Redmond giant has been accused of not complying with the ruling since February 2011, when Windows 7 Service Pack 1 was launched. Up to 28 million users who bought a PC with the latest patches pre-installed may not have seen the browser ballot window.
This display was supposed to be offered to consumers until 2014. This was after European competition authorities claimed the company was trying to maintain its dominance of the browser market.
"We take compliance with our decisions very seriously. And I trusted the company's reports were accurate. But it seems that was not the case, so we have immediately taken action," said EU competition commissioner, Joaquin Almunia.
"If infringements are confirmed, Microsoft should expect sanctions."
In a statement, Microsoft admitted it had fallen "short in its responsibilities" in updating Windows 7 Service Pack 1, blaming the lack of browser choice screen on a "technical error".
"While we believed when we filed our most recent compliance report in December 2011 that we were distributing the BCS [browser choice selection] software to all relevant PCs as required, we learned recently that we've missed roughly 28 million PCs running Windows 7 SP1," said the statement.
"While we have taken immediate steps to remedy this problem, we deeply regret that this error occurred and we apologise," it added.
Microsoft could face a fine of up to 10 per cent of its worldwide revenues (approx. $7 billion) should the case go against the company.
Alan Davis, senior competition partner at legal firm Pinsent Masons, said the EU is likely to take a dim view of Microsoft's oversight.
"Microsoft will have an uphill battle to persuade the Commission that fines shouldn't be imposed as [it] will also want to send out a deterrence message to other companies about how seriously they take compliance with commitments," he said.
"The Commission has also announced that it is going to strengthen the monitoring and enforcement of commitments in these types of cases in the future, including the requirement to have a stronger role for monitoring trustees, something that is already common in merger investigations."
