(FORTUNE Magazine) – In more innocent times, people often left their front doors unlocked. The odds of a burglar trying the front door on any given day were small, especially in rural communities.

In more innocent times, when people made Net connections via dial-up modems, the odds of being hit by a digital burglar were infinitesimally small, even though most computers, by default, leave their front doors unlocked--in fact, they have the equivalent of bright neon OPEN signs.

But with the rising popularity of cable and DSL modems, which create an always-open, high-speed conduit to the Internet, home computers are more vulnerable than ever to digital doorknob rattlers and network attacks. Cable and DSL modems give the crackers more time to cycle through their dirty tricks.

The villains who might attack your system range from teenagers with time on their hands to corporate espionage artists. They can wipe out your hard disk, swipe your financial information, or impregnate your machine with parasites that attack other machines by remote control. One morning, I logged on to a computer I help administer and engaged in a chatty conversation with a 17-year-old Israeli who had helped himself in through a system vulnerability. He offered to help me patch it.

Feeling insecure? Good. You should. All the major PC operating systems are insecure, despite years of warnings from security experts. While Apple's older Mac OS was relatively safe, because of its lack of exploitable resources, the new OS X may be more susceptible, even though Apple has, in effect, covered the windows and removed the front stairs for most services. Reports indicate that Microsoft's forthcoming Windows XP has better built-in protection than previous versions of Windows.

Home and small business users can defend themselves. The moat of choice is a personal firewall. A firewall is a protective barrier, typically a mix of hardware and software, or software alone, that examines each piece of data coming in and out of a network and blocks suspicious activity. A firewall limits access to machines inside a network, deters casual probing for weaknesses, and alerts system managers to patterns that might be attacks.

The best software-only personal firewall solution I've found for Windows machines is Zone-Alarm Pro 2.6, which can be installed and configured with a minimum of fuss. Most PC owners can use large, friendly buttons to change settings. The program automatically identifies local networks, peculiar software behavior, and outside attempts to gain access to the computer. The program offers clear, well-written on-screen messages with suggestions for how to proceed, as well as more sophisticated controls for advanced users. ZoneAlarm Pro 2.6 costs $40 and can be downloaded from www.zonelabs.com. Zone Labs also offers a free version of ZoneAlarm for home and nonprofit users.

Macintosh users can turn to Norton Personal Firewall. The program requires little configuration and is also easy to use. Norton Personal Firewall for Macintosh costs $70, or $100 in a bundle called Norton Internet Security, which includes Norton antivirus software.

Hardware firewalls usually cost a lot and are difficult to set up. But a new generation of home gateways offers affordable protection for an entire home network. One of the easiest and least expensive of these is the Farallon NetLine Broadband Gateway. The NetLine sports two Ethernet ports, one for the broadband connection to a router or DSL or cable modem, and the other for your local network. Its browser-based configuration utility allows settings to be chosen or changed from any machine on a local network. You can configure the NetLine to make machines on the local network invisible to the outside world and to let some machines have more or less exposure than others. If you're sharing files or running a Website, this is important. Farallon NetLine Broadband Gateway is $130, or $300 with an option for wireless 802.11b networking.

The best defense against a doorknob rattler is to brick up your PC door entirely. Steve Gibson's Shield's Up! test allows you to run simulated probes against your own computer (www.grc.com). If you can pass Steve's test, you can enjoy the precatapult serenity of the castle dweller.