Speaking from many years of experience as IT in the Windows and Mac world: backup, backup, backup.
I Ghost all of my Windows computers when they have a fresh install of Windows, then a second Ghost when the user data, apps etc are installed. And I do a Ghost anytime we do any installs or updates of any major kind.
On the Mac side, Carbon Copy Cloner or SuperDuper! are my weapon of choice.
Image your drives, put those images safely somewhere for easy access in case of emergency.
Always backup your treasured photos and music and documents often to many different sources. Never trust anything you get form anyone online. I can't tell you how many times someone in my family or in our office has received mail from a hacked friend or business associate that has a virus, malware or other nasty attached and they just open it right up without thinking twice.
No computer is completely free of these nasty things. No antivirus works 100%, in my experience, most anti-virus apps let bad things through. We've tried most all of them here and even with 2-3 different apps running in the background, malware, viruses etc still get through.
Sadly, until they start making it a crime and actually punishing people for making these things, they will keep doing them in droves. Your only recourse is to backup often to multiple sources and use basic common sense when receiving files.