Date published: 4/24/2012
As per Microsoft:
'This update to Microsoft Security Essentials is part of the standard product update release cycle. This service update contains product enhancements to improve performance and user experience. We have improved on Microsoft Security Essentials' Automatic Remediation, which will automatically quarantine highly harmful threats without prompting the user to take action as well as minor bug fixes in set up, malware remediation and other areas.'