Depends on which computer I'm using, currently my ultrabook:
Panda Cloud Antivirus for on-the-clock system protection+light memory footprint+background antivirus definition updates all using the power of cloud computing which has made all of my computers protected from any type of virus for 3 years now. If I download a file no matter what type of file or what type of browser I use, it automatically scans/verifies for its safety in the background, even rar files with files in them. Best anti-vir. software out there IMO.
Malwarebytes Anti-Malware for a supplemental program that I use once monthly for any potential virus that wouldn't be detected under the radar for most anti-viruses, even the software that I use. I've only received two in my lifetime that Panda couldn't detect
And Windows Firewall for firewall'd protection.
And for some reason I also have MS Security Essentials but that was through a windows update accident and idk how to remove it safely. I guess the only benefit is that it does daily scans without my knowledge.
Had to delete the Norton/McAffee bloatware on this ultrabook because it was taking over 200,000+K memory on just existing as processes and adding my boot time to 30 seconds. Happened with my Dell desktop gaming pc 5 years ago, happened with my netbook when I got it new 3/4 years ago, and had it happen to my ultrabook when I got Novemberish.