Microsoft may have retired Windows XP, but one of China's leading security vendors is trying to keep the OS threat-free, and rolling out protection software to hundreds of millions of users in the nation. On April 8, Microsoft officially ended support to the OS, meaning that Windows XP users will no longer receive security updates from the company. In preparation, Chinese security vendor Qihoo 360 has been developing its own free "XP Shield" software for Chinese PC users.