From hijacked PCs that spew spam to denial-of-service attacks that crash Web servers, cyber-crime means billions of dollars a year in lost revenues and productivity. And no computer user is safe. "It's not if," says Kevin Mitnick, "it's when are you going to get hacked."

Mitnick should know. The former hacker perpetrated a series of high-profile corporate break-ins in the 1990s -- and served five years in federal prison for it. Once the FBI's most-wanted cyber-criminal, Mitnick is now one of the world's most sought-after tech security consultants. "A few years back, companies spent more on coffee than on security," he says. Now, they make security their top priority, hiring Mitnick to break into their systems, expose their weaknesses, and teach them how to protect themselves.