A Santa Claus worm is attempting to trick America Online, Microsoft MSN, and Yahoo instant-messaging users into clicking on a file that delivers unwanted software to a victim's computer. <A HREF="http://news.zdnet.com/2100-1009_22-6002790.html?tag=zdnn.alert">The IM.GiftCom.All worm</A> attempts to dupe IM users into thinking an acquaintance has sent them a link to a harmless Santa Claus file, according to a security advisory. People who click on the file will see an image of Santa, but what they are less likely to notice is a so-called rootkit being installed onto their system.