Many years ago, a firewall was an application that protected a computer from unsolicited external connections. If computer 'A' tried to connect to computer 'B', but computer 'B's firewall wasn't configured to accept connections from computer 'A', then the connection was refused.

These days however, with the advent of Spyware, Key Loggers, Trojans and general malware the humble firewall has become much more...

Now, not only does the firewall have to protects a computer from unsolicited external connections coming in, but it must now guard against unrequested internal connects trying to get out.

Spyware and key loggers for example can be installed on your machine, not only through downloads from the internet, but also from applications introduced on portable disk drives, USB drives etc. These can monitor your key presses and send the information (passwords, credit card details etc) out to 'listening' computers.

While virus scanners and other malware detection applications can detect these things before they attempt to send the information out, it doesn't hurt to have a firewall in place just in case.

Windows XP and Vista both come with Firewall protection built in, however these are very basic and only offer protection from unsolicited incoming connections. You will need a 3rd party firewall to pretect you from unsolicited outgoing connections.

One of the best FREE firewalls around is Comodo Firewall Pro.