Users of Facebook and other social networking sites have being warned to watch out for a new computer virus spread via a Christmas card video. Sky News UK reports Facebook users are most at risk from a variant of the Koobface virus, which has been disguised as a Festive greetings video, hosted on a YouTube site. The virus works by posting malicious links on Facebook wall pages, inviting users to click on the Christmas card videos. However, any attempt to play the video turns over control of the PC to the attackers.