Connecting a computer to the Internet without a firewall is like leaving a car unlocked with the keys in the ignition. Without a firewall, your PC can easily be hacked and sensitive data stolen. In addition, a cracker (or Internet vandal) could damage the operating system or use your machine to launch further attacks against other PCs connected to the Internet.

Users with broadband connections, such as cable modems and ADSL, make particularly juicy targets. There are a couple of reasons for this.

First, every computer connected to the Internet must have a unique IP (Internet Protocol) address and broadband users usually either have a fixed address or one that is assigned for very long periods of time (ie, a "leased" IP address). That makes it easy for a cracker to repeatedly return to the same computer and wreak further mischief.

Second, users of broadband connections are more likely to be connected to the Internet for very long periods, if not permanently. That, plus the high-speed nature of the connection, means that crackers are more likely to zero in on users with cable modems or ADSL.