Windows XP will reach the end of its support cycle on April 8th, 2014 and now Microsoft also reveals that it doesn't guarantee that it will continue to provide new antivirus update for the Windows XP version of the free Microsoft Security Essentials program beyond that date.
In a note sent to ZDNet, Microsoft said it "will not guarantee updates of our antimalware signature and engine" after April 8th. It sounds like the company, in theory, could make a decision to continue its antivirus updates after support for Windows XP ends but is preparing XP users for the worst. The statement added:
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.
If you still run Windows XP on an Internet-connected machine, you're highly recommended to upgrade to Windows 7/8.1 asap. There are good, free antivirus tools though, such as Avira, but for the best line of defense against online threats you really need a modern operating system.