The point of the scam was to sell worried computer users a product called Personal Antivirus, a fake "scareware" product that bombards victims with popup ads until they either hand over their credit card information or somehow manage to remove the program.

When the complaints started pouring in, the Times first suspected that the ad had been unauthorized, and pulled third-party advertisements from the site. But on Monday spokeswoman Diane McNulty confirmed that the ad had been submitted directly to the company's online ad department.

"The culprit masqueraded as a national advertiser and provided seemingly legitimate product advertising for a week," she said via email. "Over the weekend, the ad being served up was switched so that an intrusive message, claiming to be a virus warning from the reader's computer, appeared. "

Technology executive Troy Davis was hit with the ad after he clicked on a Times story about Dubai on Saturday night. After his antivirus software warned him not to visit the article, he performed an analysis of the site and discovered that the Times was allowing advertisers to embed an HTML element known as an iframe into their advertisements. This gave the criminals a way to include embedded Web pages in their copy that could be hosted on a completely different server, outside of the control of the Times.

Apparently the scammers waited until the weekend, when it would be hardest for IT staff to respond, before switching the ad by inserting new JavaScript code into that iframe. That code redirected Davis's browser to the Web site that served a pop-up ad designed to look like a Windows system scan that had found security problems on his system.

It was, of course, all just a fake.