An Internet firewall is a piece of software or hardware that helps screen out hackers, viruses, and worms that try to reach your computer over the Internet. If you are a home user or small-business user, installing a firewall is the most effective and important first step you can take to help protect your computer. It is important to have a firewall and antivirus software turned on before you connect to the Internet.

If your computer is not protected when you connect to the Internet, hackers can gain access to personal information on your computer. They can install code on your computer that destroys files or causes malfunctions. They can also use your computer to cause problems on other home and business computers connected to the Internet. A firewall helps to screen out many kinds of malicious Internet traffic before it reaches your system.

Some firewalls can also help to prevent others from using your computer to attack other computers without your knowledge. Using a firewall is important no matter how you connect to the Internet — dial-up modem, cable modem, or digital subscriber line (DSL or ADSL).

A firewall will not make your computer 100 percent safe. However, a firewall provides the most effective first line of defense. You should install a firewall first, and then add other security measures, such as critical software updates from Microsoft Update and antivirus software. You can use Automatic Updates in Windows XP to help make sure you are installing the available updates.

A good free firewall would be ZoneAlarm.