Tablets such as the iPad continued to cut into traditional PC sales during the holiday season, and the launch of Windows 8 did little to lift the worldwide personal computer market, according to two separate research reports on fourth quarter PC shipments.

In a preliminary report this morning, Gartner said PC shipments totaled 90.3 million in the quarter, down 4.9 percent from the same quarter a year ago. IDC’s numbers show a steeper decline,with PC shipments down 6.4 percent to 89.8 million.

Both firms say lackluster Windows 8 machines from the major manufacturers deserve part of the blame. Gartner analyst Mikako Kitagawa also offers this explanation of the dynamic taking place between PCs and tablets in the home.

“Tablets have dramatically changed the device landscape for PCs, not so much by ‘cannibalizing’ PC sales, but by causing PC users to shift consumption to tablets rather than replacing older PCs. Whereas as once we imagined a world in which individual users would have both a PC and a tablet as personal devices, we increasingly suspect that most individuals will shift consumption activity to a personal tablet, and perform creative and administrative tasks on a shared PC.”

Microsoft’s big goal with Windows 8 was to capture more of those tablet sales by making the new operating system work across a variety of devices. The PC numbers don’t include “media tablets” such as the iPad, according to the firms.

Here are the Gartner and IDC charts for worldwide shipments by vendor.

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