These downloaders are constantly changing, meaning that by the time a virus definition is written to stop them, a new variant has been released.
For Symantec Mail Security for Microsoft Exchange (SMSMSE) version 7.5.4 and earlier please see the following article:
For more details on many of these attacks seen in the wild, see:
For more information on a comprehensive defense, please read the Connect article Support Perspective: W97M.Downloader Battle Plan
If Macros are not needed during normal business operations, consider blocking Macros from the internet entirely using a Group Policy Object. This article from Microsoft contains details on how to enable a GPO to block internet based Macros. If the Macro cannot execute, the end user cannot become infected, regardless of whether the original document was detected as malicious by AntiVirus solutions.
Enable Advanced heuristics detection. This technology has been effective at blocking many of these Downloaders: