As rivals complain ever louder over Microsoft's move into the security applications space, antitrust analysts predict that the company may soon be defending its business practices to government regulators once again.

European regulators have already begun to warn Microsoft against trying to thin competition in the security market via the addition of security measures in Windows Vista, due in November.

In the United States, rivals in the security applications market, including longtime Microsoft partners McAfee and Symantec, have begun to publicly express concerns over the software maker's future business plans as it builds a wide variety of security features into the next-generation operating system.

McAfee contends that at least two elements of Vista, PatchGuard and Windows Security Center, will limit the ability of third-party vendors to integrate with Vista. Lack of integration could put McAfee and others at a competitive disadvantage as Microsoft builds its own security applications into Vista, McAfee officials said.

The last time Microsoft went on the defensive was in the late '90s, over the inclusion of its Internet Explorer Web browser in Windows. "These arguments over security and Microsoft's use of its position in the operating system market to enter a new space are the same things we heard disputed in the com-pany's earlier antitrust battles," said Melissa Maxman, partner in the antitrust group of Baker Hostetler, in Washington.

On Oct. 3, Microsoft attorneys filed a new appeal over a multimillion-dollar fine the European Commission imposed on the company in July. The commission imposed the fine because Microsoft failed to meet the terms of the EC's 2004 antitrust ruling against the company.

Microsoft executives appear to view the notion of defending the company's growing security interests as ironic. The most frequent criticisms leveled at the company's products over the years, specifically Windows, relate to the many vulnerabilities in the operating system that allow computer viruses and other IT-based threats to propagate.

As Microsoft moves its security strategy forward, the software maker will focus on reducing vulnerabilities and on improving the user experience, said Stephen Toulouse, Microsoft's security program manager. "Customers ... have told us they want [Vista] to be more secure from an engineering standpoint and to have more security features and functions onboard," Toulouse said.

McAfee and Symantec have complained that Microsoft is using PatchGuard to deny third-party software makers access to the 64-bit version of Vista's kernel. They say Microsoft has fallen short on its promise to provide partners with the so-called development keys needed to build products that work as effectively as their existing applications, which are allowed to access the kernel in other versions of Windows.

Toulouse disputes the charge, pointing out that developers from both companies have been living and working at Microsoft's headquarters in Redmond, Wash., where, he said, they have been provided with full-time support for product development.

Blocking kernel access in 64-bit Vista was a security measure needed to counter emerging IT threats such as rootkit viruses, Toulouse said. Even Microsoft's own products won't access the software's core for security-monitoring purposes, he said.

Still, Microsoft must prove that its future sales hinge on the addition of security features that appear to put other companies at a disadvantage, according to Kieran Shanahan, a Raleigh, N.C, attorney who won an $89 million antitrust settlement against Microsoft in 2004.

"Hopefully they've learned from past encounters that a lot of people will be watching them and that they will not be allowed to use their market position to cram additional products down customers' throats," Shanahan said.