Firewall

Firewall can either be a piece of software or hardware. Firewall software is used to determine what incoming traffic is allowed or needs to be blocked from your computer. In order for this to occur your firewall has to look at every piece of information that tries to enter or leave your computer. If a site is trying to contact your computer and is not recognized by your firewall it will be blocked. You can customize what sites you'd like to have blocked, allowed or filtered and what products are allowed to run, and for advanced users what ports are open. 

Service Pack 2 for Windows XP has a built in firewall which is automatically enabled. Versions of Windows XP without Service Pack 2 also have a firewall but you must manually enable it. CCS highly recommends that you install the Service Pack 2 update if you have not already done so, because the Service Pack 2 firewall does not have the same capabilities as the versions before it. 

For Windows 95/98FE/98SE/ME users should install 3rd party firewall such as Zone Alarm, which is available to home users at no cost, since these operating systems do not come with a firewall already installed.