Microsoft is pushing an out-of-band Critical Update that fixes issues with their AntiMalware Protection Engine. This affects Security Essentials, Windows Defender, Endpoint Protection, etc. The bug allows for remote code execution from hackers and allows them to take control of your PC. More details can be found in Microsoft Security Advisory 4022344.
This is also reported (albeit, briefly) at US-CERT.
Good grief! Whose watching the watchers?