I've come to the conclusion that AV software gets more attrocious the more you pay for it or the more it requires advertising every 5 minutes on television. They push it on you via scaremongering every day at least once.
HOWEVER, I've been using Microsoft's free security essentials package for Windows 7 for about 2 years. It never pokes you in the eye, never lets a single thing through and doesn't screw your system resources. It just keeps out of your way. As I said, it's $0 which is how much it should cost and is supplied by the vendor which knows their own security problems the best.
With respect to Linux or MacOS X, I never have installed an AV package ever.
I like MS Security Essentials because, basically, I've already said "I'm willing to trust Microsoft for my system software." Norton, Symantec, McAfee, they all are terrible companies that I'd happily keep off my hard drive given the choice; when I'm using Windows, I've already accepted MS. Plus, since MS knows their OS better than others, they've managed to make a security system which doesn't bring a brand new machine to its knees.
The only trouble I've ever had with MSE is that if you edit your hosts file to block the Facebook "like" button, MSE will pop up a warning and delete the www.facebook.com entry.
I've had this happen, too. But after the first time I told MSE that it was a false positive, it never bothered me again about the hosts file. Good doggy.
HOWEVER, I've been using Microsoft's free security essentials package for Windows 7 for about 2 years. It never pokes you in the eye, never lets a single thing through and doesn't screw your system resources. It just keeps out of your way. As I said, it's $0 which is how much it should cost and is supplied by the vendor which knows their own security problems the best.
With respect to Linux or MacOS X, I never have installed an AV package ever.