The best, completely free antivirus is a good head on your shoulders.
Don't download viruses.
If, for whatever reason, you do accidentally or unintentionally download one.... ffs don't run it.
They cannot infect you if you do not run them.
I never install an antivirus program on my machines because of the cpu usage.
Believe it or not, but McAfee, AVG, and Norton all slow down your computer quite a bit with running process trees (sometimes more than spyware would).
If your computer does not already have herpes and has a clean bill of health I would suggest just not download harmful shit...
If you INSIST on installing some sort of anti-virus software....
Choose an antivirus that does not want to run all the time, wants to control everything you do, and/or performs scheduled scans automatically (or, if you can toggle settings somewhere... give it the least amount of responsibility). That way, you can scan when you want, without suffering the vital cpu usage.
YOU are the key to your computer running smoothly and efficiently!