Earlier this month, we reported that Microsoft would stop providing updates for Microsoft Security Essentials on Windows XP on April 8, the same day that it will cease providing security fixes for Windows XP. The company has now altered its stance and will produce signature updates for Security Essentials until July 14, 2015.
This change, it says, is to help organizations complete their migrations. Of course, using that rationale, the company should extend Windows XP's support until the heat death of the universe.
While it will provide updated signature definitions, the company warns that its research "shows that the effectiveness of anti-malware solutions on out-of-support operating systems is limited." In other words, it's hard to provide a robust anti-malware system when hostile code can penetrate processes and the kernel willy-nilly just through attacking the browser.