When asked, Eschelbeck acknowledged that 25 samples was a tiny fraction of Webroot's database of tens of thousands of malicious code samples. He also acknowledged that it may be possible for Microsoft or other competitors to pick samples of malicious code that would evade Webroot's Spy Sweeper product, given advanced knowledge of how Spy Sweeper's detection features worked.

"Nothing's impossible," Eschelbeck said.

The purpose of the study wasn't to make invidious comparisons between the two products, Eschelbeck said, but to raise questions about the detection capabilities and management of the Windows Defender product as Microsoft expands its profile as an enterprise and consumer security software vendor. "It's important to leave the interpretation up to individuals," he said. "People need to make their own conclusions about it."

Eschelbeck said Microsoft updates Windows Defender's spyware definitions weekly -- far too infrequently for the fast-moving malicious code scene.

Webroot, which is venture-funded, was an early pioneer in the antispyware software space and is one of the leading sellers of antispyware software to consumers. However, the company's prospects have been hurt by Microsoft's and the company's decision to offer Windows Defender as a free download.