The research firm says PC sales worldwide were down 8.6% between the second and third quarters. Gartner numbers say that sales were down 8.3% as compared to the same period last year.

The numbers are surprising despite an already conservative outlook exposing vulnerability of PCs to the threat of other devices and the loss of mindshare among buyers who until recent years have flocked to back-to-school promotions in the third quarter for PCs, IDC says.

Another factor is the global economy; people have less money to spend and buying a new PC is pushed down the priority list, says Roger Kay, president of Endpoint Technologies Associates. "Overall economic performance - GDP and consumer confidence - have more to say about PC sales historically, and the economy is affecting the market in general," he says.

If buyers are holding off in order to get a glimpse of Windows 8 before making a decision, the bounce after the operating system is available Oct. 26 could be modest, he says. As a practical matter, consumers buy whatever is on the shelf. If they're buying a Windows PC, and Windows 8 machines are what's on the shelf, they'll buy it, he says. "It's not a voluntary adoption," he says. "The voluntary adoption is corporate."

He says retail stores will convert entirely to Windows 8 by the end of the fourth quarter since they spent much of the third quarter selling down their Windows 7 inventory.