One-third of anti-malware products fail to secure Microsoft's Windows Vista Business Edition, according to security research company Virus Bulletin.
Out of 54 anti-malware products tested, 19 failed Virus Bulletin's security test for the software.
Problems included a failure to detect a number of viruses known to be in circulation, false alarms produced on clean files from major software houses and faults in product design and stability.
John Hawes, Virus Bulletin's anti-Malware test director, said, "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."
The most notable problem illustrated in Virus Bulletin's monthly test was the instability of products when put under pressure.
"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," said Hawes.