A trojan a day keeps the security away

My incoming email is getting flooded with virius and trojan files.  I even got one attached to an invoice from a vendor the other day.  If NAV is able to find and quarantine these things upon scanning, why won't it find and quarantine these things before I become infected?  Happens every single day.  If NAV isn't supposed to do what I want, can someone recommend virus protection that will?