German test lab AV Test has performed a comparison of business-class security solutions. The test focused on Windows 7 due to its ubiquity, and pitted 30,000 known samples of malware against enterprise endpoint security products used in businesses.
The conclusion is that while most tools had a detection rate of 100% or slightly below that, Microsoft's Security Essentials tool scored horrendous with a detection rate of only 73 percent on real-world testing and 87 percent on the real-time test. The poor detection rate of Microsoft's Defender/MSE security products is not new but these tools are popular as they're available for free.
On one hand a poorly performing anti-virus solution that is installed by default is definitely better than no protection at all, but on the other hand it provides a false sense of security as so many pieces of malware slip through.