“Taking the disruption into the courthouse was a brilliant idea and is helping the rest of the industry to reconsider what actions are possible, and that action is needed and can succeed,” said Richard Perlotto, director at the Shadowserver Foundation, a nonprofit group that tracks data about tools used for online fraud and forms of computer crime.

Mr. Perlotto and Microsoft said they did not see civil legal action against people who commit online crime as a replacement for law enforcement action, which can result in much stiffer criminal penalties. “We equate this to a neighborhood watch,” Mr. Boscovich said.

Jose Nazario, a senior security researcher at Arbor Networks, an Internet security firm, said that Microsoft’s record against botnets had been a “mixed bag” and that some of its gains were only temporary. After an earlier action against a botnet known as Waledac, for example, the software behind it was modified slightly to create a new botnet.

“You can take out a botnet, but unless you take down the coders and put the clients behind bars, they’re just going to go ahead and do this again,” Mr. Nazario said.

The computers that make up a botnet are usually conscripted without the knowledge of their owners, who unwittingly infect their machines after clicking on links in legitimate-looking e-mails for things like security updates from Microsoft and notices of tax refunds from the Internal Revenue Service. Clicking those links takes users to Web sites that exploit security holes in their browsers or other programs on their computers.

Criminals use the holes to install malicious programs that siphon personal information from the infected computers, like online bank account passwords and credit card numbers. They can also harness the infected machines to send millions of e-mail messages to other users on the Internet, including scam messages that help propagate the botnet. Sometimes botnets are rented to clients to send spam messages advertising products like counterfeit pharmaceuticals.

On Friday, Microsoft was attacking its most complex target yet, known as the Zeus botnets. The creators of Zeus offer their botnet code for sale to others and, depending on the level of customer support and customization of the code that clients require, charge them $700 to $15,000 for the software, Microsoft said in a lawsuit filed in federal court in Brooklyn on March 19.