Microsoft Malware Protection Center
In terms of detections by operating system, Windows XP continues to be the most common target, chalking up over three quarters of the detections across all platforms. Windows Vista and Windows 7 are relatively unchanged from the May report.
However, the authors of these attacks have not been resting. Just under a month ago, we became aware of a new variant of Alureon that infects the Master Boot Record (MBR) instead of an infected driver. While this new variant did not affect 64-bit machines, it had an inert file called ldr64 as part of its virtual file system. More recently, we discovered an updated variant that successfully infected 64-bit machines running Windows Vista or higher, while rendering 64-bit Windows XP and Server 2003 machines unbootable.
http://blogs.technet.com/b/mmpc/archive/2010/08/27/alureon-evolves-to-64-bit.aspx
