This month's Malicious Software Removal Tool (MSRT) takes on one of the largest botnets currently active worldwide," wrote Microsoft spokesman Vincent Tiu in a blog posting, the day the update to the software removal tool was released. "Win32/Srizbi has been accused of being responsible for a huge chunk of spam e-mail messages sent in the years after its discovery," he added. "We hope to make a positive impact with the addition of Win32/Srizbi into MSRT.