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Firewalls

Without a firewall, your computer is operating under an "open door" policy. Bank account information. Passwords. Credit card numbers. Documents and photos that you don't want to share with the world. They are all available to anyone with bad intentions and basic computer skills. Hackers can get in, take what they want, and even leave open a "back door" so they can turn your computer into a "zombie" and use it to attack other computers.

Every minute that your computer is connected to the Internet, either through a dial-up (modem) connection or through a broadband (DSL or cable) service, it is at risk.

A firewall is a piece of software that monitors all incoming network traffic and allows in only the connections that are known and trusted.

You could manually grant or restrict access to each of the 65,535 ports available under the Internet Protocol. Every time you add a new program that requires Internet access, you would need to determine which port(s) it uses, and reconfigure your computer accordingly. You've likely got better ways to spend your time.

Firewall software takes on this burden for you, allowing access to the ports you need open, and closing off those you don't. It also makes your computer "invisible" on the Internet; if hackers can't find you, they will have a hard time attacking you.

One of the oldest and most popular product lines is from Symantec, with products sold under the name of Norton. 

Here are links to some free firewalls, if you prefer to save a few dollars:

Comodo Internet Security  (includes firewall and antivirus)

(recommended)

ZoneAlarm

And, for Windows 98:  Jetico Personal Firewall v.1

There are other firewalls available, and no particular product is necessarily the best for a particular consumer.  Please examine the features of any product before purchasing to insure that this is the one for you.