One of the many inevitabilities that accompany a retiring OS, is that software developers will also stop supporting the obsolete operating system, and that includes developers of antivirus software. Usually, there is not a lot of advance warning regarding the anticipated ending of software support from those developers. However, Microsoft has today announced that their antivirus, Microsoft Security Essentials, will be ending support for XP after July 14, 2015. You can bet they will not be alone.
You can read the full announcement here:
Microsoft has announced the Windows XP end of support date of April 8, 2014. After this date, Windows XP will no longer be a supported operating system. To help organizations complete their migrations, Microsoft will continue to provide updates to our antimalware signatures and engine for Windows XP users through July 14, 2015.