In the early days of computer attacks, when bright teens could bring down corporate systems, the point was often to trumpet a hacker's success. No longer. In today's murky world of digital viruses, worms, and Trojan horses, the idea is to stay quiet and use hijacked computers to flood the Internet with spam, spread destructive viruses, or disgorge e-mail to choke corporate systems. Not only can networks of these compromised computers be leased or sold, experts say, they are becoming more valuable as the number of vulnerable computers slowly shrinks.

That's a major reason that turf wars are emerging among hackers. Besides infiltrating computer systems, the viruses are now also designed to kill any other competing viruses in those systems. These skirmishes have gone on - quietly - for several years. Last week, for the second time in a little over a year, they exploded into public view. A worm dubbed Zotob infected computers at major media outlets, industrial companies, and San Francisco International Airport.

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