Now, Lanstein and FireEye were chasing their mightiest target to date, the Web’s most sprawling and advanced spam machine, called Rustock—pusher of fake pills, online pharmacies, and Russian stocks, the inspiration for its name. Over the past five years, Rustock had quietly—and illicitly—taken control of over a million computers around the world, directing them to do its bidding. On some days, Rustock generated as many as 44 billion digital come-ons, about 47.5 percent of all the junk e-mails sent, according to Symantec, the computer security giant based in Mountain View, Calif. Although those behind Rustock had yet to be identified, profits from it were thought to be in the millions. “The bad guys,” is what Lanstein had taken to calling them.