Microsoft's Live OneCare security software has failed tests which check how well it spots and stops malicious programs designed to attack Windows.
The OneCare software has failed two independent tests
OneCare was the only failure among 17 anti-virus programs tested by the AV Comparatives organisation.
Microsoft's software only spotted 82.4% of the 500,000 viruses that the independent group subjected it to.
The test is the second in less than a month that Microsoft's anti-virus software has failed.
Live OneCare is Microsoft's flagship security program that, like many other anti-virus products, is designed to help PC users keep their machine clear of malicious software.
Austria-based AV Comparatives carries out quarterly tests of top anti-virus programs to find out if they have kept up with the growing mass of viruses circulating online.
The tests see if anti-virus software can spot a variety of malicious programs including macro and script viruses, worms, backdoors and trojans.
The majority of programs tested, 14, got an advanced pass or better from AV Comparatives; two got a standard rating and OneCare failed.
OneCare fared particularly badly against so-called polymorphic viruses which regularly change their configuration in a bid to fool security software.
"Microsoft OneCare performed very low in the test, and did not reach the minimum requirements for participation," wrote Andreas Clementi, senior tester at AV Comparatives, in the report about the tests.
A Microsoft spokesman said: " We are looking closely at the methodology and results of the test to ensure that Windows Live OneCare performs better in future tests."
He added that Microsoft was trying to: "determine whether any learnings from these tests can be used to improve our services as part of our ongoing work to continually enhance Windows Live OneCare to ensure the highest level of protection and service that we can provide our customers."
The spokesman also pointed out that Windows OneCare had been certified by ICSA Labs and the West Point Checkpoint system.
In early February, security magazine Virus Bulletin revealed that OneCare failed to spot all the viruses it tested the software against.