Another Microsoft security rival posted results Wednesday of tests claiming that Windows Defender, Microsoft's antispyware software, blocked only a portion of the malware thrown at it.

The trials, conducted by Enex TestLab but paid for by Melbourne-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, Webroot Software 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.

Over the past year, various analysts have pegged stand-alone antispyware software as a dead end, primarily because Microsoft gives away Defender and bundles it with Vista. Other factors accounting for the prediction include the move toward comprehensive suites from the likes of Symantec and McAfee that boast spyware detection as well as antivirus scanning.

Windows Defender recently came under increased scrutiny for a flaw in the malware-scanning engine that drives it and all other Microsoft security software. The bug, which was disclosed Feb. 13 in the month's regularly-scheduled patch update, was fixed for most users -- including those running Windows Vista -- by automatic upgrades that actually began arriving on PCs in late January.

A Microsoft spokesman could not be reached for comment.

Defender can be downloaded free of charge for Windows XP or Windows Server 2003 from the Microsoft Web site.