About a third of anti-malware products tested by Virus Bulletin failed to secure Windows Vista Business Edition SP2 with several "crumbling" under the pressure of a heavy attack.
Of the 54 products tested by the vendor, 19 failed to reach the standard required for VB100 certification, the latest Virus Bulletin report said.
The VB100 tests assessed anti-malware products against a test set from the WildList - a list of viruses known to be circulating on computers that is maintained by a group of anti-virus researchers.
Products had to detect 100 percent of the malware contained in the WildList test set and "not generate any false alarms when scanning a set of clean files" to achieve VB100 certification.
Some of the products tested by Virus Bulletin in August produced false alarms on clean files from the likes of Corel, Roxio and Adobe.
Most notable this month was the remarkable level of instability under pressure noted in many of the [anti-malware] product," Virus Bulletin anti-malware test director John Hawes said.
"While our tests do put unusual strain on products, it is clearly important that security software should continue to function under pressure, and should not crumble in the face of heavy attack.
"Flaky behaviour will certainly not instill a sense of security in users, and developers need to ensure their quality control is thorough and comprehensive, to keep their users properly protected at all times."