In many cases, the scammer asks the user to open Windows' events log, which records significant events on the PC, including program errors. Richmond noted that the log typically contains scores of such errors, which may look alarming to many users.

Of the people who received such calls, 22% fell for the scam, Microsoft said.

Most who were duped suffered some kind of post-call financial loss, which Microsoft claimed averaged $875 per victim. Among the losses, people cited compromised passwords, balky computers, identity fraud and cash pilfered from their bank accounts.

"Criminals have proved once again that their ability to innovate new scams is matched by their ruthless pursuit of our money," said Richard Saunders, the director of one of Microsoft's public relations teams, in a Thursday statement.

Although the scam currently targets Windows users -- and for now is limited to English-speaking countries -- there's nothing to stop criminals from expanding their scheme. "Presumably, when these guys smarten up a little bit ... they could pull exactly the same trick [on Mac users]," said Paul Ducklin, Sophos' head of technology for its Asian and Pacific division.