The results of a recent AV-Test involving twenty-one different antivirus software running on Windows 8/8.1 have been published. Unfortunately, Windows Defender Antivirus came last in the protection category. Past tests have been similarly disappointing.
In response to a WinBeta inquiry on the subject, a Microsoft Spokesperson had this to say:
The results from the latest AV comparison tests do not accurately reflect the protection that Windows has to offer or what our customers are experiencing. The extensive set of protection capabilities provided in Windows, including staying up to date with security fixes from Windows Update, in addition to advanced exploit mitigations built into the platform, and features providing active defense, like SmartScreen and Windows Defender, provide an end-to-end solution. Only when all of these capabilities are analyzed together can one truly measure Windows’ ability to defend against modern threats.
So to clarify, Microsoft is saying that Windows Defender AV works together with other Windows features such as SmartScreen and should be tested with that in mind. Even taking that at face value, test results such as these, on Windows components don’t help with Microsoft’s “the most secure Windows ever” promotion for Windows 10.