According to Microsoft pirated Windows XP product keys were used to spread the Citadel malware. Redmond isn’t claiming a full take down because of the sheer size of the botnet but, it is optimistic that the seized equipment will help it to strengthen defenses in the future.

via Microsoft, FBI takedown Citadel botnet – ParityNews.com: …Because Technology Matters.

Prior to take down of Citadel, Microsoft has been a key player in take down of other botnets like Bamital, Kehlios and Nitol botnet