In recent testing, Germany's AV-TEST Institute found that Microsoft Security Essentials only protected against 64 percent of zero-day malware attacks. As a result, the software failed to receive the institute's certification.

"The test rates each product in three categories: protection, repair, and usability," explains PCMag.com's Neil J. Rubenking. "Protection refers to preventing attack by a collection of malware samples that includes widespread malware, recent detections, and zero-day attacks. The repair score is based on the product's ability to detect already-present malware and remove it thoroughly. Products lose usability points if they significantly impact system performance or falsely report valid programs as malicious. In each category, a security product can earn from zero to six points. To receive certification, the product needs a total of 11 out of the 18 possible points."

"During October the Institute rated Security Essentials 4.0 and 4.1 at just 1.5 out of 6 in terms of its ability to protect a PC, thanks largely to the 64 percent zero-day detection rate being well below the industry average 89 percent," writes The Register's Simon Sharwood. "Security Essentials has lost AV-TEST’s seal before, with its September 2010 test failing to meet the lab’s criteria. It is the only one of 24 AV products for Windows 7 without the certification."


"Its heuristics engine has been called into question before, and a solid behavioral scanner has become critically important to anti-malware apps in recent years," notes Geek.com's Lee Mathews. "Malware is constantly evolving, and more rudimentary definition-based detection schemes simply aren’t enough (though on a positive note, MSE did manage to detect 100% of known malware samples older than 3 months)."

"It's rarely a good idea to trust one test results to base an entire judgment on, but there's no doubt that these scores are a major cause for concern not only for people who use Microsoft Security Essentials, but also because a lot MSE has gone into Windows 8 security," writes CNET News' Seth Rosenblatt. "Nobody wants to deal with a computer virus or malware infection, though, so I'd recommend that people running MSE change to another, better regarded free security suite as soon as possible. Avast or AVG have solid security reputations. The current AV-Test top-rated suite for security efficacy is Bitdefender, but the cheapest version starts at $39.95."