Microsoft Tuesday said the coordinated take-down of the Rustock botnet and follow-up efforts had purged the malware from over half of the PCs once controlled by Russian hackers.

"This shows that disruptive action [against botnets] is viable and possible," said Richard Boscovich, a senior attorney with Microsoft's Digital Crime Unit.

"Once you start taking apart the infrastructure of botnets, you drive up the cost of [botnet gangs] doing business," Boscovich added in an interview Monday. "Disruptive action is just as good as trying to arrest someone."

Since March, when Microsoft lawyers and U.S. Marshals seized Rustock command-and-control (C&C) servers at five Web hosting providers in seven U.S. cities, the number of Windows PCs infected with the malware has dropped worldwide from 1.6 million to just over 700,000 as of June 18, Boscovich reported in a blog post today.

Microsoft also released a detailed report on Rustock, the take-down effort it led, and the impact of its anti-botnet campaign (download PDF).

In the U.S., an estimated 86,000 Rustock-infected PCs in March had been reduced to some 53,000 by June, a drop of 38%. Other countries saw even bigger reductions: In India, the March tally of 322,000 infected machines plummeted by 69% to approximately 99,000 in June.

The take-down itself didn't remove the Windows PCs from Rustock control. Instead, the seizure of the U.S.-based C&C servers and Microsoft's work to snatch control of the domains that Rustock was coded to use for fallback communications, prevented the botnet from updating itself.

That in turn provided the breathing room antivirus vendors needed to issue signatures for the existing Rustock malware and users the opportunity to scrub their systems with security software.

Microsoft, for instance, has provided Rustock signatures for its Malicious Software Removal Tool (MSRT), a free utility that detects and deletes malware, since 2008.

The take-down of Rustock's communications channels effectively silenced the botnet.

Since March, the botnet -- which was once one of the largest purveyors of spam, particularly pitches for fake drugs -- has been quiet. "Botnet activity dropped abruptly to almost zero in mid-March following the take-down," Microsoft said in its report.

Prior to the take-down, Rustock was capable of sending as many as 30 million spam messages daily.

"Cleaning the users' PCs is an important part, but really this shows that a technical countermeasure along with a legal countermeasure works," said Boscovich, talking about the two-pronged approach of seizing servers and shutting down Rustock's backup communications.

And the impact goes beyond Rustock.

"The minute you take down Rustock, what does that do to those who want to send spam?" Boscovich asked. "They have to find other botnets. But if you're a botnet herder, and you just saw Rustock go down -- with years of work coding and planting malware and maintaining the botnet -- you're going to charge more. And that's an impact on spammers' cost analysis, as it becomes more and more expensive to send out spam."