A firewall is an absolutely necessary tool in every computer user's defensive arsenal, along with an antivirus utility. Firewalls are programs (sometimes hardware devices) that are designed to protect you and your system from the legions of hackers, crackers, and other evildoers who probe the Internet for unprotected PCs. Once they're in, such invaders can obtain private information or take over the computer for nefarious uses.

The problem is particularly acute if you have a full-time cable or DSL connection: Both types of connection use a limited and well-known range of Internet addresses that can be continually poked and prodded. (During a recent 24-hour period, we logged 463 intrusion attempts on a PC connected to a cable modem.) But dial-up Web users aren't immune, either.

Windows XP has a basic built-in software firewall (see Step 1), but for more-robust protection, you should opt for a commercial package. Companies like Sygate and Zone Labs offer both free and paid versions (the latter have extra features). Other vendors, such as McAfee and Symantec, offer only paid versions. Go to Editor's Roundup: Firewalls to download free, shareware, and trial editions of firewalls, and see this month's Internet Tips to read about one particular package.

Although firewalls differ, they share key features. We've come up with a list of the common steps and settings necessary to put your PC on high alert. The examples here are from Symantec's Norton Personal Firewall and Zone Labs' ZoneAlarm Pro. The exact steps for other packages will vary, so read your software's manual and online help carefully.