Vixie said most of the victims are probably individual home users, rather than corporations that have technology staffs who routinely check the computers.

FBI officials said they organized an unusual system to avoid any appearance of government intrusion into the Internet or private computers. And while this is the first time the FBI used it, it won't be the last.

"This is the future of what we will be doing," said Eric Strom, a unit chief in the FBI's Cyber Division. "Until there is a change in legal system, both inside and outside the United States, to get up to speed with the cyber problem, we will have to go down these paths, trail-blazing if you will, on these types of investigations."

Now, he said, every time the agency gets near the end of a cyber case, "we get to the point where we say, how are we going to do this, how are we going to clean the system" without creating a bigger mess than before.

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Online:

To check and clean computers: http://www.dcwg.org

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Lolita C. Baldor can be followed on Twitter at http://twitter.com/lbaldor