ZoneAlarm

Everyone knows what a virus is and that you should protect against it, but what is a firewall? A firewall does two things: 1. It blocks intruders trying to access your computer from the internet or a network. 2. It blocks unauthorized programs on your computer from accessing the internet. Okay, you see why the first thing would happen, but why the second?

One very common reason is spammers trying to use your machine as a "zombie pc". If you have a compromised computer, you may be unknowingly sending out thousands of spam emails. This happens invisibly to you with the only symptom being a slow-down of your computer and internet speed. Sound unlikely? Experts estimate that most spam is now generated from compromised computers. Simply put, you need a firewall.

http://www.zonelabs.com/