Avast SUCKS!!
i used to have it on my computer, it was too confusing. when it said "virus deleted" and i made a re-scanning, the virus was still there.
plus, you hardly un-install it (whether through "add-remove programs" or "un-install Avast".
running a scan, is there between options. i don't remember where now ...
i used to have Norton, i liked it though it took a lot of my ram (especially when i used to have 256MB Ram in my old computer lol)
I haven't tried Zone Alarm, but a friend of mine has the "Zone alarm Internet Security" package and he says this is the best antivirus.
i trust him.
so i would recommend Zone alarm, (though in free version)