Rival calls foul over Microsoft's delivering Security Essentials via Windows Update

Other security vendors, including Symantec and McAfee, declined to say whether they, like Trend Micro, viewed Microsoft's move as anticompetitive or unfair. Instead, they downplayed Security Essentials' effectiveness.

"It's clear that today's threat landscape requires more comprehensive protection than what Microsoft Security Essentials offers," said Symantec in a statement. "From a security perspective, this Microsoft tool offers reduced defenses at a critical point in the battle against cybercrime."

McAfee took the same tack.

"Options that provide an elementary level of security, including Microsoft Security Essentials, mostly rely on traditional protection mechanisms," McAfee said. "McAfee products offer not only more features but most importantly, McAfee products offer real-time protection using cloud-based intelligence to combat even the most sophisticated threats."

All three vendors scoffed at the idea that they're scared of free antivirus rivals, and by implication, Security Essentials. "We've competed against free for a long time," said Carpenter. "We've not seen [free products have] much impact on our market share."

This isn't the first time that security companies and Microsoft have butted heads.

In 2006, Symantec and McAfee complained to European Union antitrust regulators about Microsoft's decision to block them from accessing the kernel in the 64-bit version of Vista, and barring them from its new integrated security center. Microsoft bowed to the pressure, and later promised to produce APIs (application programming interfaces) that gave security vendors some access to the kernel and allowed them to mesh their product's on-screen status features with the security center.

Carpenter declined to says whether Trend Micro would consider legal action against Microsoft over the issue, but said that her company "was always looking at issues like this."

In a follow-up e-mail, Carpenter was clearer. "We're concerned that Microsoft may be using its OS-based market leverage to box out other choices. If that were to happen, it would not be good for consumers or the industry, and would warrant a second look."

Trend was aware of the Security Essentials offer in the U.K. that started last month, but Microsoft's decision to do the same in the U.S. caught it by surprise. "We work with Microsoft on a lot of levels," Carpenter said, but added that Microsoft had not told Trend Micro it was expanding the deal to the U.S.

Not all eligible U.S. users have seen the Security Essentials offer because Microsoft is rolling it out over the course of the month, the company said.

Users who decline the download and then want to block the offer from reappearing can do so by right-clicking on the Security Essentials item, then selecting "Hide update" from the drop-down menu.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@computerworld.com.

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