I'm using AVAST AV + Comodo Firewall in Win 8.1 with Spybot Search and Destroy as an extra layer of Antispyware. I also have MalwareBytes scanner installed and scan my computer once a week. AVAST is the most lightweight and best free AV option out there. Using it for last 6 years without any issues (Not a single infection till date in all the machines I've used XP/Vista/7/8 and I'm one of those who use torrent frequently). If you are in Win 8.1, please install the latest version of AVAST as the older versions have conflict and you should be good to go.
PS: I was using ZoneAlarm firewall earlier, it's awesome except has compatibility issues with 8.1. So after it is stabilized, it might become a good alternative to Comodo Firewall.