The spam-spewing Kelihos botnet has returned from the dead.
Microsoft collaborated with Kaspersky Lab to run a successful takedown operation last September. The takedown decapitated the botnet by shutting down command-and-control server nodes, directing the bots on infected computers to contact a server under the control of security researchers, rather than one controlled by the attackers.
Story @ The Register
Analaysis @ Kaspersky