McAfee Inc. Chief Executive George Samenuk said Microsoft Corp. is compromising the security of millions of computer users by backing away from years of technical cooperation with security-software makers.

The security-software maker reached out to reporters Monday and placed a full-page advertisement in the Financial Times newspaper charging Microsoft with anticompetitive behavior.

“Microsoft is using something out of their old play book in not collaborating with us,” Mr. Samenuk said in an interview.

Microsoft disputed the notion that it is freezing out security companies or promoting its own security products at the expense of third-party players. And it charged that it is McAfee and Symantec Corp. that are trying to restrict competition.

Characterizing the changes as security enhancements, Adrien Robinson, a director in the Microsoft Security Technology Unit, said the software giant remains committed to working with security companies. “Microsoft needs to raise the bar in security,” she said. “We know we can’t do this alone.”

McAfee and competitor Symantec have complained recently about changes made by Microsoft in how third-party security products must interact with the new version of the Windows operating system, Vista.

The companies say Microsoft is denying security-software makers access to the core of the new operating system, which is known as the “kernel,” that they need to provide effective defenses against viruses and other attacks on Windows PCs, by far the main focus of malicious attackers.

“The new operating system, Vista, leaves the user, leaves the consumer, less safe than ever before,” Mr. Samenuk said. Without kernel access, “our security (products) may not be as reliable as they have been for the last decade, and that hurts the consumers, and it hurts Microsoft.”