Careful internet use is also a good safety measure for your computer

Don't open unknown email attachments, don't click on suspicious links, and don't click ad pop ups if they do get through your pop up blocker. And a big rule *for some of us* don't let your husband look at porn sites

~ men! They are almost always full of pop ups and spyware, and phishing crap....
Of course this rule doesn't apply to everyone because not everyone is going to visit those places, but sometimes people just get curious and have to go look ... it's just human nature, that's why advertising works.
I don't use a firewall, and I've never had a problem from not using it. I really just can't use one because of the communication programs I use for working....a firewall intereferes and messes up the transfers. Now, if I didn't have that circumstance I would probably have the firewall on.