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You're in luck, mein freund, as several firewall vendors offer gratis versions that'll suit the lion's share of casual users. You'll have to endure frequent pitches to upgrade to the pay version, and make do without frills, but for the most part, zero dollars can buy you protection enough from the Internet's ruffians—assuming, of course, you haven't got any high-powered geek enemies whose mission in life is to infiltrate your C drive.

First, some background for the greenhorns: A firewall is the software equivalent of a bouncer, charged with deciding who gets the privilege of entering Club Your Computer. Tons of poseurs are always sniffing at your machine, trying to worm their way in through gaps in the velvet rope (known as ports). A firewall basically picks up these interlopers by the scruffs of their necks, determines that, no, they're not worthy of quaffing Yukon Jack at your virtual V.I.P. table, and boots them in the arse.

You don't mention what type of operating system you're sporting, but if it's Windows XP, you already have a firewall in place. It's not switched on by default, so you'll have to do some dreary Windows-ing—go to Network Connection, select your connection, click Change Settings, go to the Advanced tab, and enable the Internet Connection Firewall (ICF). This is the cheapest, easiest way of firewalling yourself, but it's far from Mr. Roboto's favorite—the ICF isn't easily customizable, and thus'll cause you a million headaches if you share files or play online games.

Instead, PC users should consider a freebie add-on. The easiest to use is ZoneAlarm (zonelabs.com), essentially an install-it-and-forget-it tool for newbies to intermediate users. The chief gripe folks have about firewalls is that configuring 'em is tough, but that's not the case with ZoneAlarm. It remembers which programs you let access the Internet, and it does a marvelous job of staving off unauthorized intruders. The Pro version, which adds a pop-up blocker and a cookie foiler, runs $50 a year. But honestly, unless you're going to be acting as an Unreal Tournament server, the free version'll do you right.

A slightly more powerful alternative is Sygate Personal Firewall (sygate.com). The latest version, 5.0, deftly handles a wide range of devious cyberattacks, like system-crashing IP fragmentation. The downer is the interface, which is a bit trickier to configure than ZoneAlarm's smooth-as-butter control panel. A one-year license for the Pro version costs $40, but if you're gonna go that route, you might as well pay the extra $8 to get automatic upgrades, too. No matter how conscientious you are, you're just not going to visit the corporate Web site every week to download the latest attack signatures, are you?

More advanced users looking for handouts will be happy with Outpost Personal Firewall (agnitum.com), which supports plug-ins, or GNAT Box Light (gta.com), for the home networking set. As for you Mac users, let Mr. Roboto first say "congrats," because fending off hackers just isn't going to be a huge issue round your parts. The vast majority of computer users worldwide run Windows, and since Mac OS X is based on the tried-and-true UNIX operating system, odds are your box will never become the personal plaything of a 13-year-old from Minsk. Still, it wouldn't hurt to plunk down a measly $25 on BrickHouse (securemac.com), a shareware utility that simplifies the process of configuring OS X's built-in firewall.

Once you've got your free firewall installed, it's advised that you test it for holes. Download LeakTest from the Gibson Research Corporation (grc.com), which'll check to make sure your firewall's tuned up to detect the nastiest "Trojans"—malicious code that hackers sneak onto your machine, to be exploited at a future date.

If you're still paranoid after going through all that, and you're still not willing to open your wallet, well, then Mr. Roboto ain't gonna shed no tears. Either deal with the fact that cyberspace-on-the-cheap will never be 100 percent secure, or train some carrier pigeons. Your choice.

Input questions at bkoerner@villagevoice.com.