The Windows 8 trough - the suppression of PC sales because customers want to see Windows 8 before deciding to buy it or something else - was definitely a factor in the numbers, says Ezra Gottheil, senior analyst with Technology Business Research.

"That a new operating system suppresses sales in the quarter preceding its availability is proven by history," Gottheil says. "It even happens to ."

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That said, other factors came into play, according to Gartner. "Professional PC shipments in the U.S. began slowing in the second quarter of this year, and they continued the trend in the third quarter," says Mikako Kitagawa, principal analyst at Gartner in a written statement. "The results indicate that the replacement peak may have passed in the professional sector." Plus there is a continuing slowdown in consumer PC sales, she says.

In a statement announcing its third-quarter PC sales numbers, IDC says some of the slowdown can be pinned on a "loss of mindshare" among students who historically respond to back-to-school sales.