Firewalls hide your computer or network from Internet threats and can be either hardware or software.

Hackers use programs that roam the Internet and search for open computers. They do this by sending information to IP addresses. If the IP address is unprotected, a message is sent back to the hacker. The hacker then knows your computer can be infiltrated.

Hardware and software firewalls prevent this from happening by accepting only requested information. For example, every time you type in a Web address or access the Internet, you are requesting information. If you type in my home page (www.komando.com) , a request is sent to my Web site's server.

The server acknowledges the request and sends the information, and your computer displays it. Since your computer made the request, the firewall lets the information through.

Stopping Trojans

This is the first step to protecting your computer, but it's not enough. What happens if you download a free program or e-mail attachment that includes hidden, but malicious, program called a Trojan, which requests information from or sends information to the Internet without you knowing it? Because you initiated the request to download the software or open the attachment, a hardware firewall won't stop the Trojan.

That's where a software firewall comes in. It alerts you with a pop-up message whenever a program tries to access the Internet. If it's a valid program, such as Internet Explorer or Outlook, you tell the firewall to allow access. If it's an unknown or suspicious application, you can block it.

This can get confusing. You'll be amazed at the number of programs that need Internet access. Sometimes, your music player needs access to online databases when playing CDs or MP3s. Other programs automatically log on to the Net to check for software updates.

After installing a software firewall, you'll initially get bombarded with messages. If you recognize the program name, grant it access. If you don't, deny access and then look up the name on the Internet.

Wide variety

There are a number of free and pay software firewalls. Windows XP has a built-in firewall. This firewall works much like a firewall on a router. It's able to shield your computer from hackers trying to get in, but it does nothing if you have a Trojan trying to get out.

That's where solid third-party firewalls come in. Several companies market software firewalls that are free for personal use, including Agnitum's Outpost (www.agnitum.com) and ZoneAlarm (www.zonelabs.com) . Firewalls are also available from McAfee (www.mcafee.com) and Symantec (www.symantec.com) for less than $50.

Mac OS X has a built-in firewall similar to the one included with Windows XP. If you want something better, check out offerings from Intego (www.intego.com; $59.95) and Symantec ($69.95).

Firewalls are just one part of a bigger equation. You still need anti-virus software and operating system updates.

All work in tandem to keep the predators out.