Windows XP support is ending on April 8th, 2014. Along with it will be support for Microsoft Security Essentials support for the OS. This leaves it pretty vulnerable for a lot of new attacks, as well as those that have been waiting for the deadline. Several anti-virus vendors have also pledged to stop supporting the OS. So, if you are still running Windows XP, now would be a good time to upgrade to a newer OS or get ready to unplug the machine from the Internet.
Microsoft has created a website showing some good reasons to upgrade, as well as a great countdown to the April deadline.