Why the argument is pointless
A tablet is a PC, so it's silly to talk about how it's going to kill the PC. It's like suggesting that dolphins are going to eliminate the whale population. Dolphins are technically whales, so it's not possible.
Although Microsoft has been taking some heat for the allegedly slow launch of Windows 8, Microsoft actually seems to understand the tablet-PC relationship as well or better than anyone.
Windows 8 is a bridge OS. It attempts to straddle the line between the traditional Windows-based PC we're used to, and the next-generation, touch-based, mobile-oriented devices we're moving toward. Windows 8 itself may struggle for a wide variety of reasons, but ultimately it's a step in the right direction, and indicates that Microsoft has a vision for the future.
Intel announced this week at CES 2013 that touchscreen displays will be a core requirement of the next generation of ultrabooks. Hardware vendors have inundated the market with crossover hybrid hardware like the Dell XPS 12 Convertible or the Lenovo IdeaPad Yoga that attempt to be both a PC and a notebook. Microsoft's Surface is a tablet that looks and acts like an ultrabook with the addition of one of the keyboard covers.
The line is blurred because we're talking about an evolution of the PC market, not the execution of it. The NPD tablets vs. notebooks data projects through 2017, but I predict by 2015 we'll stop considering them as separate markets.