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In December, XP PCs accounted for 11.2 percent of Windows computers sold through retail (online and brick-and-mortar operations), according to NPD. For notebooks: 13.7 percent.

NPD provided data for Windows operating systems shipped on PCs, which is what I requested—not retail boxes. NPD tracks sell-through, not ship-to. Microsoft tracks Windows licenses shipped into the channel. NPD's numbers track Windows PCs sold to customers. Sell-through more accurately reveals market trends.

"The XP number is almost all netbooks," Stephen Baker, NPD's vice president of industry analysis, told me earlier today. "Before August, XP was zero."

Whoa, that's a dramatic change and further evidence that Microsoft was right to mainly blame netbooks—or mini-notebooks, as some analysts call them—as major reason for recent Windows revenue declines. During fourth calendar quarter, which is Microsoft's fiscal second, Windows Client revenue fell 8 percent year over year; operating income declined by 13 percent.