AV software is one of those topics everyone typically has an opinion on

In general, Paper is right, many AV solutions have some serious overhead, especially the legacy stuff. As for me, I use Eset on all 3 of my machines and IMHO it is one of the top 1-2 solutions. And, I know free sounds cool and everything but...I don't mind spending money on something as fundamental as AV software - then again, I tend to hang out in some pretty sketch neighborhoods.
Oh, BTW, in general it's not a good idea to have multiple AV products loaded on your computer at the same time. On a more general note, unfortunately the more you write about your current problem, the less it sounds like there's gonna be an easy fix. Last - and only marginally helpful - comment, following an orderly process in tracking down a problem like this is usually a good thing...