After using many popular softwares I used I chose: Avast! AV Home, Ad-Aware Free, (I currently have Ad-aware Plus, but I think the free one is more worth it, the only difference is the "Ad-Watch", I didn't find it much useful than scanning) Windows Firewall(installed with my system), and Windows Defender (also installed with my system)
But I do have AVG Free on my older HP system and the following: Spybot S&D, Ad-Aware SE, Sygate PF. AVG runs smoother on slower systems I guess. I use Avast! because the interface looks a lot better.