There are many free antivirus and malware protection programs out there. However very few of them are worth installing on your computer.
1. Comodo - In terms of loading speed and the app that consumes less resource, I'd recommend comodo. It has decent virus definition update rate and detects most of the common viruses. Free version has few limits but gets the work done.
2. Avast - They offer 1 year free key which you can renew every year. Though they are now limiting most of the features for paid version. Still I'd say avast is a good antivirus software out there.
3. Panda - It doesn't have much popularity due to other antivirus are performing quicker and have better definition upgrade rate. However this app is also one good free antivrius solution.
4. Bitdefender - They have popular anti malware product and that performs much better than their antivirus solution. However, the bitdefender offers good option for those who are looking for free antivirus.
My personal suggestion is to go with either Comodo or Avast for free antivirus solution.