As part of the May 2013 Updates Release, Microsoft has issued a silent update for the Malware Protection Engine. A vulnerability in the Malware Protection Engine has been reported to allow remote code execution, so it's important to take special notice of this update.
The vulnerability only affects the x64-based versions of the Malware Protection Engine, but both x86 and x64 systems received an engine update as part of the monthly patch cycle.
Read all about it: Microsoft Security Advisory (2846338)
A few in the community (on the NT Sys Admin email list), this morning, are reporting that the services for Microsoft Security Essentials will not start on some of the computers on their networks. After further investigation, those computers are also running an additional security application from Trend Micro.
I assumed it was a given, but I think it's important to highlight that it's not considered a best practice to run multiple security products on the same computer. Installing and running multiple security products on a computer will cause both to have to compete for resources, causing a serious performance degradation. But, even more than that, its regularly reported that by having two or more different security products running on a computer, one will cancel the other out or cause a multitude of problems that are hard to diagnose.
Bottom line: Don't do it.