Your computer is runing windows2000 or an earlier version, and there are no firewall installed. Or you are using windows XP or Vista, and you do not really trust the built-in firewall and you would like something more secure.

A good, free solution might be ZoneAlarm from CheckPoint

ZoneAlarm from checkPoint is a free firewall software perfectly scaled for the home computer. It just does what every firewall software should do : it prevents inbound and outbound connections from your computer, except on some specific ports that you have defined, or for some specific software that you have defined.

The user interface to allow or blok programms or ports is very simple : when a software or port is accessed from outside, or need to access the outside, a pop up windows appears and ask you wether this should be allowed or block. Then you decide, and this decision is remembered in the future. Done.

 It has taken many awards, including from PCMagazine and NetworkWorld, and has a reputation for being unattackable.

You can download it from here, and alo pay a visit to the ZoneAlarm page.

 Want more ?

I would not recommend it for business (except if your business has just 1 computer), as there are no centralized way to manage configuration for al clients on whih it is installed. also, it does not offer any roaming option : it is not possible to change the configuration depending on wether you are at the office or on the move.

Please also see related post Free Home computer security – Secure your home computer