Microsoft's free Security Essentials antivirus software identified 98 percent of over half a million malware samples, an accuracy rating an independent testing company called "very good" today.

Germany-based AV-Test.org tested Security Essentials, the free software Microsoft shipped Tuesday, on Windows XP Service Pack 3 (SP3), Vista SP2 and the final code of Windows 7, against two different collections of malware, said Andreas Marx, one of the firm's two managers.

The first test put Security Essentials in the ring against more than 3,700 viruses, Trojans and worms culled from the most recent WildList, a collection of threats actively attacking computers. "All samples were successfully detected and blocked during our on-demand and on-access tests," Marx said in an email today.

The second test sicced Security Essentials on a much larger set of malware. Of the 545,3444 malware samples in that collection, Microsoft's software nailed 536,535, resulting in what Marx characterised as a "very good detection score" of 98.4%.

In a follow-up test of adware and spyware detection - Security Essentials also includes anti-spyware scanning - Microsoft's software spotted 12,935 out of 14,222 samples, for a 90.9% accuracy rate.

This is the second time that AV-Test.org has run Security Essentials through the mill; when Microsoft launched a limited preview in June, the group tested the beta. Then, the free software also breezed through the WildList , spotting every sample in the 3,200-plus set.

Security Essential's final version also successfully identified and deleted all 25 rootkits AV-Test.org threw against it, Marx said.

But there were some things that Microsoft's program had trouble handling. Most security software now includes an ability to sniff out malware by the way it behaves, often by using heuristics-based scanners that don't rely on specific "fingerprint" signatures to match against a potential threat. Security Essentials lacks any such technology.