Microsoft has dealt a fatal blow to what was once of the world's largest botnets. Redmond and partners including Symantec, Shadowserver Foundation, the University of Washington and others set out earlier this year to tackle the Waledac botnet in an effort dubbed "Operation b49."

The group filed a legal complaint in February, prompting a temporary restraining order against 277 domain names believed to be involved with Waledac. Last week, a federal judge granted Redmond legal ownership of all but one of those domains, allowing the software giant to cripple Waledac permanently.

Systems infected by Waledac in February

At its peak, Waledac sent some 1.5 billion spam emails per day from tens of thousands of computers around the globe. As many as 90,000 systems were compromised at one point, but that number fell to 64,000 by early July and around 58,000 unique IP addresses were affected as of August 30.

Microsoft is contacting ISPs to inform customers that their machines are infected and cleanup is well underway. More importantly, this could set a precedence for future cases, making it easier to behead other nefarious web operations.