If you're reading this on a Windows machine and you don't know if you have a personal firewall installed and running, then stop what you're doing and take care of that right now. At the very least, turn on the Windows firewall. This feature is available in the PC control panel, and enabling it only takes a few seconds. You can come back and read this once you've done that. I'll wait.

OK. You're back. Now breathe a big sigh knowing that you may have prevented a worm infestation that truly would have ruined your day. Feel better? I know I do. Let's continue.

You've noticed by now that I believe personal firewalls are vital on Windows machines. While you may also have a firewall on your network, that's not enough protection. The reason is that while a firewall between you and the Internet will keep worms and hackers from entering down that path, it will do nothing to prevent attacks that come from within your network—or through paths that aren't blocked by the company firewall. And those can ruin your day just as effectively.

There are a lot of personal firewalls options on the market to choose from. Most make similar claims, but there are differences, and depending on what you're doing with your computer, those differences can matter. But then, so can the cost, and that can matter a lot since some choices are free.

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