The trials, conducted by Enex TestLab but paid for by Australia-based vendor PC Tools, concluded that Defender was able to sniff out and block 46.6 percent of the sample spyware set when running its quick scan, and 53.4 percent using the more time-intensive full scan. Defender ships as part of Windows Vista but is also available as a free download for Windows XP.

PC Tools' flagship product, Spyware Doctor, fared better, according to Enex, which said the software's quick scan blocked 83.3 percent of the spyware and its full scan stopped 88.7 percent.

Last month, Boulder, Colo.-based Webroot Software Inc. ran similar tests on Defender, using a spyware sample of its own creation, and claimed that the Microsoft title barred the door against just 16 percent of the sample malware.

PC Tools took aim at Webroot as well as Microsoft. "While we agree with the overriding conclusion that Vista security is lacking, [Webroot's] approach fundamentally contradicts the laws of statistical analysis, and clearly creates a bias result," Simon Clausen, PC Tools' CEO, said in a statement. "By hand-picking the sample set, it is easy to return results showing whatever you want. It would even be possible to show Vista had 0 percent blocking ability."

Clausen said that Enex Testlab, not PC Tools, choose the spyware and adware to throw against Defender and Spyware Doctor.