Firewalls

A firewall is a form of Internet defence. Just as it is recommended that a user should keep up-to-date anti-virus and anti-spyware software running on their system, it is also critical to maintain a strong firewall. A firewall's purpose is to monitor and block any perceived threat from the Internet to the local system, eg: hackers.
Firewalls can be software or hardware based, or a combination of the two. An example of a hardware firewall could be a router, and a software firewall could be Windows Firewall or ZoneAlarm. Windows Firewall is included in Windows XP, but offers only basic firewall facilities.
ZoneAlarm is freeware and can be downloaded and installed for personal use. Once installed, ZoneAlarm can be configured to permit Internet access as required by the user, for example, a browser's access to the Internet. Comodo Firewall Prois another free firewall product.
Another alternative is Sunbelt Personal Firewall. You can download a demo version of this and run it free for 30 days, but you need to pay a small fee if you want to keep using it after that.