Microsoft will not guarantee updates of our antimalware signature and engine after the XP end of support date of April 8, 2014. Running antivirus on out of support operating systems is not an adequate solution to help protect against threats. Running a well-protected solution starts with using modern software and hardware designed to help protect against today’s threat landscape. In addition, Microsoft recommends best practices to protect your PC such as: 1) running up to date antivirus, 2) regularly applying security updates for all software installed, and 3) using modern software that has advanced security technologies and is supported with regular security updates.
Microsoft's message about the advantages of consistently updated software is well-covered ground. By continuing antivirus updates, they would just be enabling behavior they have done their best to discourage. Users who insist on continuing to run Windows XP can shift to one of the other free products (such as Panda, AVG and Avira).