With the holiday shopping season behind us, it's clearer than ever tablets are cutting deeply into traditional PC sales, even after the launch of Windows 8.

According to Gartner, PC sales fell by 4.9 percent in the fourth quarter of 2011, compared to the same quarter a year earlier. Lenovo and Asus actually managed to grow their PC shipments by 8.2 percent, and 6.4 percent, respectively, but they couldn't make up for sales declines for other PC makers, including a massive 20.9 percent dip in shipments for Dell.

The problem, according to Gartner analyst Mikako Kitagawa, is that consumers have become less reliant on PCs for casual use. So while families may keep a shared PC at home for work and creative tasks, they're buying new tablets instead of replacing their aging laptops.

“There will be some individuals who retain both, but we believe they will be exception and not the norm,” Kitagawa wrote in a press release. “Therefore, we hypothesize that buyers will not replace secondary PCs in the household, instead allowing them to age out and shifting consumption to a tablet.”

Microsoft's Windows 8 is supposed to stave off the PC's decline by combining a new tablet-friendly interface with a traditional desktop. To take full advantage, some PCs such as Lenovo's Yoga convert into tablet-like forms, and some tablets such as the Acer Iconia W510 come with attachable keyboard and trackpad docks. The idea is that users can have the benefits of a tablet without sacrificing productivity.

But the launch of Windows 8 hasn't exactly been smooth sailing. Despite a launch date of October 26 for Windows 8, the most interesting hybrids and convertibles were in short supply during the holidays, due to supply chain issues.