Last September, I wrote about fake virus scan pop-ups that you sometimes encounter while using a web browser, sometimes known as “scareware.”

What I didn’t cover was a class of malicious software known as “ransomware,” the fake virus scanner’s more violent cousin. The difference?

• Scareware: tries to trick you into purchasing useless software and probably installs spyware, adware and other malware.
• Ransomware: poses as a virus scanner, but locks up your computer and forces you to purchase useless software to unlock your computer. Also likely installs a bunch of other malware, in addition to the fact that you’ve just given criminals your credit card number.

It’s kind of the difference between a con artist and a mugger, I guess.

There’s no real way to tell offhand whether a fake virus scan pop-up window is scareware or ransomware. It doesn’t really matter—you don’t want it either way. The same rules for prevention apply in both cases.

Both start the same way: you visit a website and a window pops up that tells you your computer is infected with a virus. The pop-up almost always has an “OK” and a “Cancel” button. Do not click on either of these, because they both install the malware.

You can click on the “X” in the upper-right corner of the window, but I don’t even like to do that. I use “CTRL-ALT-DEL” to force the browser to close. I think the Mac version of “CTRL-ALT-DEL” is “Command-Option-Escape.”

After I’ve shut down the browser, I run a virus scan and a spyware scan. It’s sort of a pain and it takes a while, but too many people value convenience over security, and they end up paying for it. There are very few instances in which it’s not possible to find something else to do while your virus scanner runs. You don’t have to be on the Internet 24/7, you know.

Now, I’m not one to tell anybody what brand of web browser to use, but I will say one thing on the topic: since I switched from Internet Explorer to Firefox with the NoScript plug-in, I haven’t had a single scareware window pop up. I’m not telling you what to do. I’m just sayin’.

Also, I know it costs money, but you cannot afford not to do it: install some good antivirus software, keep it updated and keep your subscription current. Norton, McAfee, Kaspersky; I don’t care which one you use, just use something. No, it’s not super cheap, but if you’d rather shell out $79 to unlock ransomware than spend $69 on actual protection…well, in that case I think there’s just something the matter with you.

Finally, for an extra level of protection, install the excellent (and free!) Spybot Search & Destroy. Yes, right now. There is one annoying thing about this software, though, and it’s Microsoft’s fault: in Windows Vista and Windows 7, in order to run S&D properly, you can’t just click on the icon. You have to right-click the icon and select “Run as administrator.” You won’t be able to actually remove anything if you skip this step.

There’s a recent story about ransomware at MSNBC, with a video that shows the malware in action (and actually shows you how to unlock it with hacked registration codes).