Compromised U.S. computers: 210,000

Main crime use: Also called Downadup, this downloader worm has spread significantly throughout the world, though not so much in the U.S. It's a complex downloader used to propagate other malware. Though it has been used to sell fake antivirus software, this crimeware currently seems to have no real purpose other than to spread. Industry watchers fear a more dangerous purpose will emerge.