BRUSSELS -- Microsoft is working to hamstring software companies trying to overcome "inherent weaknesses" in Windows security, rival McAfee Inc. charged in a full-page ad in yesterday's Financial Times.

McAfee, Symantec, and other security software companies argue 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.

No 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 spokesman said.

The commission says it gained authority over Microsoft's additional new offering as 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.

"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 spokesman said.

McAfee's ad echoes comments by Symantec officials in a recent interview that Microsoft has withdrawn cooperation as it moves to substitute their security software with its own, giving its own product a leg-up in Windows.

They say they are denied access to the heart of the operating system through built-in software locks, 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," says the advertisement in the Financial Times.

Microsoft disagrees. ``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 informal complaints of security companies 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 center of enforcement actions in the United States, European Union, and South Korea. Regulators tried to get Microsoft to change 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 commission's 2004 decision.

McAfee's ad says 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."