Norton Internet Security 2012. I paid for it and it does things I've never seen free security software do. Scans every freakin' file coming in, traces them, does idle time scans, tells me if memory is low and I have NEVER gotten a false alert! EVER!
I've been quite happy with MSE and it's free and is updated via Windows Update so there is one less notification setting that needs to be changed. As far as CPU usage and memory footprint, all the major packages are pretty inconsequential. If your system is limited and 5% CPU usage or a few dozen megabytes are an issue for you when gaming, upgrade your system, reduce your visual settings, or run without live AV during your gaming sessions. Before I got this i7 laptop, I did all my browsing within a virtual machine running Ubuntu and ran without any AV software. Seven years of that on my old Pentium M laptop and not a single infection.
I love it because its "light touch"... but very effective. Some of the other ones recommended above I have found to be very "bloaty"... and just wont shut up... They seem to feel like they have to tell you they started up... that they updated definitions...that they identified that your internet browser history is not clear!!!
Kaspersky just seems to queitly get on with it. Its ad blocker is great... I dont realise just how much crap comes at you on some web sites until I visit on another comp... I really appreciate that
And also... I NEVER notice it doing a scan....may be beacuse I leave my comp on all day - prob plenty of idle time for it to do it... but I never notcie it impacting through scans.
I am careful - so dont expect to get warnings... but the very few I have had gives me confidence it doing a good job.