Windows Defender/ Microsoft Security Essentials.

They are decent for free. I'm always very careful that my system is updated, and I don't click any suspicious links/ files.

Unfortunately, a lot of the free Antivirus have become so intrusive. They look at your web traffic/ mail, and some go as far as to man-in-the-middle your https connections. Of course they do it in the name of "security". But I've stopped using Avast because of this reason.

Some of the paid services like ESET or Kaspersky are decent. Every time I have to deal with changing a setting in Symantech I get annoyed because it takes over control of the Windows firewall and things like that.