McAfee, Symantec and other security software companies argue that Microsoft's new Vista operating system will make it more difficult to protect customers because for the first time, they have been denied access to the core of the operating system.
Neither company has filed a formal complaint, and so far, the European Commission has taken no formal action on the matter.
"We have an ongoing dialogue with Microsoft. It's not up to us to give them a green light. It's up to them to assure full compliance" with the law, a European Commission representative said.
McAfee chides Microsoft
CEO George Samenuk and Chief Security Architect John Viega of McAfee discuss the impact of Windows Vista on security with CNET News.com's Joris Evers.
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Theas part of a landmark 2004 case, which found that the company abused its dominant position in the Windows operating system to damage rivals.
Microsoft says it may withhold shipping Vista to European Union states when it distributes the operating system next month to computer makers and companies, out of concern about enforcement action. But no decision has been made.
"Our goal is to deliver a fully innovative, secure version of Windows Vista that is compliant with EU law. We have an ongoing and constructive dialogue with the Commission on these issues," a Microsoft representative said.
McAfee's ad echoesas it moves to substitute their security software with its own, giving its own product a leg-up in Windows.
They say they are, which makes it much harder to protect.
"Microsoft is being completely unrealistic if, by locking security companies out of the kernel (core), it thinks hackers won't crack Vista's kernel. In fact, they already have," the advert in the Financial Times read.
"Partners are at the core of Microsoft's business model. We have worked closely with our security partners throughout the development of Windows Vista, and continue to do so," it said.
The echo those of other companies over the years, which charged that Microsoft illegally cut them out of their core markets.
The companies sued or were at the centre of enforcement actions in the United States, European Union and South Korea. Regulators tried to get Microsoft to get changes its business practices, but none has succeeded.
Microsoft defends its practices as proper, legal and a boon to consumers and innovation. It says that it should be able to improve Windows without harassment by governments and has made a court challenge to the.
Companies have lost their dominant positions to Microsoft, then either exited the market, ceased competing or settled their lawsuits against the software giant for sums reaching hundreds of millions of dollars.
These companies include the browser maker Netscape, the server maker Novell and the streaming media player RealNetworks.
Once Microsoft conquers a market, its innovation generally slows as it turns to focus on new, unconquered markets.
McAfee argues that Microsoft "seems to envision a world in which one giant company not only controls the systems that drive most computers around the world, but also the security that protects those computers from viruses and other online threats.
"Only one approach protecting us all: When it fails, it fails for 97 percent of the world's desktops," McAfee said in the ad.
"Computer users around the globe recognize that the most serious threats to security exist because of inherent weaknesses in the Microsoft operating system," it said.