I don't usually promote commercial products on this blog but Sophos are giving away Sophos Home free for the whole family.
Sophos Home is a full-featured computer security product that protects up to 10 personal Mac and Windows computers from malware/viruses, malicious software, and inappropriate websites. It's good to see Mac security included.
With no credit card required and no ads. You can get it from Sophos here.
Of course for total coverage you will also need tablet and smartphone security.