There's tons of free options that are perfectly adequate.
Avast, Avira, AVG, BitDefender, etc.
I've tried 3 of those. And to my experience, you get what you pay for. Had my comp completely lock up with rogue fake anti-virus programs taking over, making my computer into a brick. Only saving grace was Safe Mode and System Restore. If either of those didn't work, would have been screwed. I've had the best results with a cheaper paid anti-virus coupled alongside a free complimentary scanner such as malware bytes.
The fuck. I haven't gotten a virus since the days of Kazaa