A firewall is a software program or piece of hardware that helps screen out hackers, viruses, and worms that try to reach your computer over the Internet. If you can’t start Windows Firewall or you are getting an error, use our free tool to diagnose and fix problems.If you use a computer at home, the most effective and important first step you can take to help protect your computer is to turn on a firewall.Windows 8, Windows 7, Windows Vista, and Windows XP SP2 or higher have a firewall built-in and turned on by default. (Note: Support for Windows XP ended in April 2014.)If you have more than one computer connected in the home, or if you have a small-office network, it is important to protect every computer. You should have a hardware firewall (such as a router) to protect your network, but you should also use a software firewall on each computer to help prevent the spread of a virus in your network if one of the computers becomes infected.If your computer is part of a business, school, or other organizational network, you should follow the policy established by the network administrator.