Last week, my colleague, Jeff Fox, was perusing the New York Times online when a window popped up warning him (and many others) that his computer was “at risk” and in need of immediate protection. The window looked legitimate, very similar to his own security software. But Jeff knew better. From years covering all the nasty maladies that can infect a computer, he recognized the popup as a “malvertisement.” He immediately closed his browser and, to be safe, ran a virus and spyware scan of his hard drive. Had he followed the popup’s prompt, seeking protection, he might have exposed his computer to an online attack.

So-called malvertisers have become adroit at smuggling their software into online ads, where they can entice or frighten users into following harmful links in hopes of remedying a supposed problem with their computer. The tactic NYTimes.com readers saw—a malvertisement warning them of infection—is known as “scareware,” as it hopes to snare less savvy folks than Mr. Fox. (Another coworker mentioned to me recently that his elderly father, unaware of such ploys, is notorious for clicking on malvertisements, which infect his computer.)

Perhaps in response to the attack on such a well-trafficked, reputable site as NYTimes.com, Microsoft last week filed five lawsuits against alleged malvertisers. Although the guilty individuals remain anonymous, Microsoft hopes that the suits, which are directed at several front companies (named Soft Solutions, Direct Ad, qiweroqw.com, ITmeter INC. and ote2008.info) will uncover the culprits.

To protect yourself, be vigilant. Be wary of any popup windows that look fishy, especially if they appear alarmist. When closing such a window, carefully click the “X” in the corner and not anywhere inside the window itself. Sometimes the popups make it exceedingly difficult for you to get rid of them. In those frustrating cases, it’s best to quit your entire browser.

For more on staying safe online and avoiding clever tricks like malvertising, see our free Online Security Guide. —Nick K. Mandle