What I'm about to say isn't directed at you.  So please understand this is a generalization but it applies to thousands if not millions of people.

Nothing is idiot proof.

I've had computers with Norton, AVG, AVAST and others but all have had a multitude of Trojans and other virus programs. 

Why?

Because people go to sites such as porn sites or hacker/cracker sites.  Because people open emails that are spam but laced with a virus.  Because people forward stupid pictures, audio and video with a virus to their friends who just open them and never check them for a virus.  Because a popup says you have a virus and you need to download such and such a program to fix it.  Because people download extra toolbars for their browsers.  Bargain buddy, search buddy and others.

Because people don't have a firewall other than the windows firewall and people don't UPDATE their virus protection.

For all these reasons and more nothing works.

We've been virus free for years just using Avast or Avg and either ZoneAlarm or Comodo.  I have a few other programs but mostly it's just we don't click on things that I've mentioned.