Pick a friendly Linux distribution. My personal preference being Ubuntu: The price is right, the installation is a no-brainer, the default setup is easy to use. Because Linux (like UNIX) is set up so that regular common-or-garden users cannot make permanent changes to the system you would have to strain hard to be infected by malware -- and there is far less of it aimed at you.

The only CON: Installation takes more than 20 minutes; about an hour, but after the first 10 minutes you can leave it to continue automagically.

Mike