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.