Microsoft Corporation has made a significant investment over the past few years researching and combating malicious and potentially unwanted software, and in developing technology to help customers mitigate the security risk that it creates. As part of this investment, Microsoft created a dedicated antimalware team that is responsible for researching malicious software (or “malware”) and potentially unwanted software. In addition, this team is responsible for the release and maintenance of the Microsoft® Windows Malicious Software Removal Tool (MSRT) and Windows Defender.

This report focuses on the first half of the 2006 calendar year (from January to June) [H106] and expands upon a white paper Microsoft released in June 2006 entitled MSRT: Progress Made, Lessons Learned (http://go.microsoft.com/fwlink/?linkid=67998). Compared to the MSRT paper, this report includes more recent results based on a significantly expanded set of data sources. Using data derived from several hundred million Windows users, this report provides an in-depth perspective of trends in the malicious and potentially unwanted software landscape.