Ramblings from another nerd on the grid
A firewall is a critical first line of defense against many types of malware. Properly configured, it can stop many kinds of malware before they can infect your computer or other computers on your network. Windows Firewall, which comes with Windows Vista, is turned on by default and begins protecting your computer as soon as Windows starts. The Windows Firewall Control Panel is designed to be easy to use, with several configuration options and a simple interface.
More advanced than the Windows Firewall in previous versions of Windows, the firewall in Windows Vista helps protect you by restricting other operating system resources if they behave in unexpected ways—a common indicator of the presence of malware. For example, if a component of Windows that is designed to send network messages over a given port on your PC tries to send messages via a different port due to an attack, Windows Firewall can prevent that message from leaving your computer, thereby preventing the malware from spreading to other users.