Avast and Norton are HUGE system hogs
Zone Alarm since the latest version Nothing not even spyware has got through it even blocks all the maleware facebook sends to your computer.
Norton™ AntiVirus 2010
on my dell xps and my netbook running W7 i have Norton 360 and cannot tell that they are running.. No Hog here.. now i will agree that untill 2007 there software was a hog. they have come along way since. i am well pleased.
KAV hands down... no other anti virus beats it... i swear to everything awesome, if you buy KAV you will be protected like a triple wraped tool in a dirty you know what... trust me... KAV = best, updates virus signatures ALOT quick 10 second updates that start automatically that update whenever new viruses are released, or new strains of trojans are released. super fast detection. I love it.
Because no-one will admit to having crap software on their computer, their own installed antivirus is always the best.
You will never get a true answer so read as many independant reviews as possible to help make your mind up.
BTW, I have AVG suite and it beats anything out there
Microsoft Security Essentials sucks!
I like Kaspersky and Nod32, Kaspersky is for the advanced users, you can set a lot of things, and Nod32 in another way, is easier to use!