Worldwide PC shipments declined by 8 percent in the third quarter, in advance of the Windows 8 release, according to numbers released by the IDC and Gartner research firms this week. Looked at in the most favorable light for Microsoft, the dip suggests that the industry, businesses and home computer users are waiting eagerly for the launch of Windows 8, clearing out inventory and holding off on new purchases until the new operating system debuts.

Either that or they’re already turning their attention — and budgets — to iPads and other alternatives.

“PCs are going through a severe slump,” said IDC analyst Jay Chou in a news releasing announcing the numbers. “The industry had already weathered a rough second quarter, and now the third quarter was even worse. A weak global economy as well as questions about PC market saturation and delayed replacement cycles are certainly a factor, but the hard question of what is the ‘it’ product for PCs remain unanswered. While ultrabook prices have come down a little, there are still some significant challenges that will greet Windows 8 in the coming quarter.”

The chart above shows the trends over the past two years, with traditional PCs, primarily running Windows, still shipping far more units than tablets. However, the PC shipment trends are flat overall, in contrast with the steady growth in shipments of iPads and other tablets.

This, in a nutshell, is why Microsoft is trying to expand the reach of Windows 8 with its new tablet-friendly interface, willing to take the risk of alienating some traditional computer users in the process. The Windows 8 launch on Oct. 26 represents a new era for Microsoft, in a variety of ways.

In the meantime, these shipment trends don’t bode well for Microsoft’s earnings report a week from today.

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