Norton, actually. It'll lag your computer like crazy and literally block out anything the slightest virus can get on, though. Not good if you have a gaming computer.
PC Magazine has a collection of reviews of free anti-virus. See the link, below.
So far no one has posted a download link in their answer. If someone does, please don't click it. They probably mean well, but you shouldn't trust anonymous software downloads, especially for anti-virus software.
Basically that depends if you are using windows then there is Microsoft Security Essentials which is free anti-virus which works awesome than any other anti-virus.(In windows 10 there is no need of anti-virus as windows 10 have inbuilt software named as Windows Defender)