I've used zone alarm for years, no problems with it. Pro isn't needed. Anytime you open a program that needs to connect to the internet (like those CD players that check the music databases to show CD track info), a pop up alert will ask if you want to connect. This way any types of hidden codes that want to communicate out (like those hidden in HTML email spam) will send out that alert.
Once installed, test out if the hacker world can still get into your system by visiting https://grc.com/x/ne.dll?bh0bkyd2
and using the 'Shields Up' test, the 'Probe my ports' test and the 'Leak test'. This site is pretty cool, has lots of info on keeping your system safe, important if always connected via cable modem.
Marc
"The reason angels can fly is that they take themselves so lightly." --GK Chesterton