I've just finished fixing a computer where a virus had disabled Microsoft Security Essentials (and had altered IE's proxy configuration in order to hijack the user's web searches). MSSE was still there, but it didn't detect the virus if I started a scan from the command line, while its graphics interface would close shortly after launching it. It was the same problem that
these guys were having almost two years ago. Like them, I had to install another antivirus to remove the 50+ instances of the virus that were lying undetected on the hard drive.
I hope that MS will fix the accuracy of MSSE, for an antivirus is essential to have on Windows (at least for non-technical users) and what the competition offers tends to be heavy, infested with nag screens and unwanted features. Somehow this reminds me of the days of DOS 6, when Microsoft had added a nice built-in antivirus to the OS (MSAV), but then stopped upgrading it, and removed it altogether from later versions of their OSes.