A personal firewall is another way to protect your computer. You may not be aware of it, but your computer is listening on a large number of ports. These ports are affiliated with specific services that your computer either uses or provides. For instance, in order for you to look at this web page, your computer is allowing tcp traffic on port 80 (this can be changed, but the default port for unencrypted http traffic is port 80). If you were to block port 80, you would be unable to view http web pages. However, certain programs shouldn't need to communicate via that port. For instance, Solitaire should not need to communicate across port 80. If it attempts to do so, that might be a sign of a problem with your system. In addition to monitoring communications from your computer, a good firewall will also monitor communications directed towards your computer.
So what we need is a way to identify what programs are communicating via the network, and possibly what ports they are communicating over. This is what a personal firewall will do.
If you have Windows XP SP2 or later, you already have a personal firewall. You can access it by opening the Security Center (Start -> Settings -> Control Panel -> Security Center). If the personal firewall is active, you will see an image like this:
If your firewall is turned off, you should turn it on. To do so, click on Windows Firewall under Manage security settings for: and then click On (recommended) and click OK. The firewall graphic in the Security Center should now match the one above. Close the Security Center.