Firewalls
What is a Firewall?
Firewalls are a key part of keeping networked computers safe and secure. A firewall is a piece of software or dedicated hardware which inspects network traffic passing through it, and denies or permits passage based on a set of rules.
Basically, data sent over networks and the internet is sent in as a series of packets. In addition to the data, these packets have a header which contain, among other things, the IP address and Port. A firewall filters these packets, by blocking ports that you have not explicitly opened to traffic. There are some common ports, such as 80/TCP (used for transferring web pages) that are always left opened. Having a firewall active is the second crucial step in keeping your computer secure, especially if connected to the internet through a permanent broadband connection.
My network has a fairly common configuration, with a hardware firewall that sits between the router on my internal network and the rest of the internet. As well, each computer on the network runs its own software firewall. I will leave hardware firewalls out for now, and instead focus on software firewalls. Installing and maintaining one should help to keep your computer secure.
Firewall Usage
I tried several free firewalls, and my top recommendation is Comodo firewall. I found it very easy to install and use. When traffic was detected, the pop-up gave me all the information I needed, and once traffic I was aware of was all given permission, it ran silently and effectively in the background. Another excellent choice is Sunbelt-Kerio Personal Firewall. However, it does begin showing a nag screen after 30 days. That is enough for me to not use it, but if you can put up with them, it is an excellent firewall.
After you first install the firewall software, you will notice that the firewall will begin to detect network traffic. You will then get the option of allowing or blocking the traffic. Most of what you see you should be able to allow, especially if you have recently done a scan for spyware. Comodo will tell you the source of the traffic (for example, Windows, firefox.exe, iexplore.exe). If you recognize the source, than allow the traffic. If you are not sure, you can type the source into a web browser such as Google and find out what it is. After you have been running a firewall for a while, these detections will become much less common as your firewall will have learned what network traffic to allow.