I have a business that runs Outlook Express and recently they got hit by the latest 2013 fake AV scareware. They do have some type of version of Symantec AV installed but it has failed despite always being updated.

My questions are:

What is a free AV or protection service that would plug in to Outlook Express and protect from threats and block them from executing? 

Are there certain ways Outlook Express should be configured and setup for added protection?