When connected to the internet or to a network, the risk for a computer to get infected is more possible. Many data files are sent by hackers in order to depict computer connected. In this way, they detect vulnerability of and way to access the computer.
This is one of the main purpose of installation a firewall in view to protect a computer from network intrusions.(or protecting a local network of attacks from the Internet).
Also, a firewall can control access to network applications on the machine. Moreover Trojans can open a breach in the system to allow for ownership of the remote machine by a hacker. The firewall identifies suspicious connections of the machine and even prevents attack.
ZoneAlarm is a personal free firewall for protecting constantly a machine on network. It is very simple to configure and installed and its high level of protection is recognized by the protection of personal computers.