Recently, I received an email with attachment in my Yahoo Mail Inbox.  Yahoo Mail indicated that the attachment contained no virus after scanning it with Norton Anti-Virus.  So I proceeded to download the zip file.  Thankfully, I have avast! anti-virus installed.  avast! warned me that the zip file contained Trojan Horse.  I quickly canceled the download.  I highly recommend avast!anti-virus.  There is a free version for home users.