Firewalls limit access to you and from you computer. A firewall can block unwanted connections to and from your computer, which leaves your computer less prone to computer hackers.

Windows 2000 does not come with a built-in firewall. There are several free (freeware) or cheap (shareware) firewalls available online on sites like http://download.com/ or http://tucows.com/. ZoneAlarm from ZoneLabs (http://zonelabs.com/) is another popular firewall software.

Windows XP has a built in firewall, which is easy to enable. Here is how:

1. Click Start > Control Panel > Network Connection
2. Right click Local Area Connection and go to Properties from the pop-up menu
3. Under the Advanced tab, place a check next to "Protect my computer ... from the internet."
4. Click OK to apply changes.

or you may choose a firewall of your own listed above under Windows 2000.

By default, none of the ports on Mac OS X are open, but a firewall is still recommended. Mac OS X has a built-in firewall if you are running remote access services such as FTP, SSH, Telnet, etc.

There are several third party products available from sites like http://versiontracker.com/ and Apple's very own Product Guide at http://guide.apple.com/