I love Kaspersky adn they're my favorite. But I didn't enjoy paying for it yearly. So I decided to go with a free solution and the one I currenly use is Avast like many others have also mentioned. The ads are a bit annoying the the newer version of the antivirus, but they can easily be disabled in the options.
If I had to recommend another paid option, I would recommend NOD32 by ESET, which is right next to Kaspersky and depending on who you talk to they will recommend it one way or another. Another good free option would be Avira, which I'm currently giving a test drive on my main rig, but for some reason I keeps giving me the "closed umbrella" icon, indicating that virus protection is currently disabled and everytime it try to re-enable it, it freezes or does do anything...but other people have said good things about it!