The European Commission has fined Microsoft a record €899 million (£680 million) for taking until late last year to honour the 2004 anti-trust ruling against it.
The latest EC punishment means Microsoft has been fined roughly €1.7 billion (£1.3 billion) over the issue. The commission had already fined the company €777.5 million (£590 million) - €497 million (£377 million) in the original ruling plus a further €280.5 million (£212 million) for non-compliance.
Microsoft finally came into compliance with the 2004 ruling last October. Commissioner for Competition Neelie Kroes said the latest fine - the biggest yet - is for non-compliance up to 22 October 2007.
"The Commission's latest fine is a reasonable response to unreasonable actions by Microsoft," Kroes said, describing the company's tardy response as "a clear disregard of its legal obligations".
Over the past four years, the software giant has repeatedly tried to avoid complying with part of the Commission's ruling that ordered it to detail communications protocols used by its Windows server operating system, so that other manufacturers could build systems to interoperate smoothly with Windows.
The latest fine also punishes Microsoft for failing to license those protocols to open-source software developers at what the EC views as a reasonable price.
Microsoft said it is reviewing the commission's action. "The commission announced in October 2007 that Microsoft was in full compliance with the 2004 decision, so these fines are about the past issues that have been resolved," it said in a statement. "We are focusing on steps that will improve things for the future."
Anticipating new fines, the company announced last week that it will make the biggest effort yet to help other companies make their products interoperate with its most popular software, including Windows and the Office suite which includes Word, Powerpoint and the Outlook email program.
The Commission reacted with scepticism, pointing out that it had heard similar promises before, and added that the move only addresses one of two new anti-trust probes into Microsoft's business practices, which were opened last month.
Both new cases build on the same legal arguments that underpinned the 2004 ruling: one looks into the company's failure to share essential interoperability information in Office. The other is looking into whether the tying of Internet Explorer to Windows amounts to an abuse of anti-trust law.
The second case is based on the part of the 2004 ruling that dealt with the tying - or bundling - of Media Player to Windows. Microsoft was ordered to sell a second version of Windows with Media Player stripped out. The company complied, but the remedy was deemed useless because the second version was sold at the same price as the version bundled with a media player, and no one bought it.