In my opinion, Win 8.1 is in and of itself a virus or plague on the unsuspecting public. Beyond that, though, I just use the stock Windows anti-virus. It lets in all the same viruses that every other major anti-virus company lets in, and it is free.
I'm with buttonsHT on this, though. I'd rather not have any virus protection. Unfortunately, if you use the internet at all, or download any software at all, you need it. (I suspect that buttonsHT actually has had malware on his PC at some point and never realized it. Several well-known software companies were using some ad related software in the early 2000's that was later found to be malware. They stopped as soon as they found out, but still, there was an awful lot of people that used that software.)