Both complaints accuse Microsoft of failing to change its business methods since the company was found guilty of anti-competitive behavior in 2000 and breaking European Union competition law in March 2004.

The US complaint has been filed by a Tangent, a small Burlingame, California-based hardware vendor that says it is a Microsoft certified partner, but has still been caused significant harm by Microsoft’s exclusionary and restrictive practices.

In a filing with the US District Court for the Northern District of California, Tangent claimed Microsoft continues to engage in anticompetitive conduct and has caused it damages by increasing, maintaining or stabilizing the price [Tangent] paid for Microsoft’s operating system software above competitive levels.

Tangent claimed relief under the Sherman Act and cited recent US Department of Justice and European Commission criticism of Microsoft’s failure to provide third parties with relevant technical documentation, as evidence that the company continues to prevent interoperability with non-Windows systems.

Tangent also claimed Microsoft is guilty of exclusionary conduct in bundling its .NET platform with Windows, and in withholding information that would enable third party applications from accessing Microsoft Office documents.

Other examples of anticompetitive behavior cited by Tangent include bundling of Outlook with Office and Active Directory with Windows Server, as well as the bundling of Windows Media Player and Windows Media Server with its desktop and server operating system respectively.

Microsoft did not respond to a request for comment on Tangent’s complaint, other than to acknowledge that it was being reviewed, but was more forthcoming in responding to a fresh complaint lodged with the European Commission by the European Committee for Interoperable Systems (ECIS).

ECIS is a front for IBM and a few other competitors who constantly seek to use the regulatory process to their business advantage. When faced with innovation, they choose litigation, said a Microsoft spokesperson in response to reports of a complaint relating to the way Windows interoperates with Office applications.

ECIS was formed by IBM, Nokia, Oracle, RealNetworks and Red Hat, amongst others, in April 2005 to provide assistance to the European Commission in its fight against Microsoft. The membership roster is also reported to include Sun, Linspire, Opera and Corel.

While the new complaint could pose problems for Microsoft with its appeal against the Competition Commission’s antitrust remedies due to be heard by the European Union’s Court of First Instance between April 24 and April 28, the company is unmoved by it.

We have come to expect that as we introduce new products that benefit consumers, particularly with the kind of breakthrough technologies in Office 12 and Windows Vista, a few competitors will complain, Microsoft’s spokesperson added. We will respond quickly and comprehensively to any requests for information from the Commission on this complaint, but no such requests have been received so far.

Suggestions of fresh complaints against Microsoft in the US and EU have been brewing for some time. Last week the US DoJ revealed that it is investigating antitrust complaints related to Microsoft’s forthcoming Vista operating system, while in September 2005 the European competition commissioner, Neelie Kroes, revealed that the EC was reviewing fresh antitrust complaints made against the company.