Do all PCs sold already have Windows on them?

Microsoft CEO Steve Ballmer announced at the company's Build conference today that more than 4 million Windows 8 upgrades had been sold since the operating system's official launch on Friday. He also said "tens of millions of copies" had been shipped to partners. Ballmer said yesterday that "preliminary demand" was higher for Windows 8 than it was for Windows 7, but offered no numbers to back up that claim at the time.

The 4 million upgrades he announced today give some idea of the OS's success, but putting the numbers in context is actually quite complicated. Aside from yesterday's remark, Windows 8 sales have not been compared to Windows 7's — that's not surprising considering the OS has only been available for a few days. Even when Windows 7 was moving enough copies to be considered the fastest-selling OS in history, Microsoft waited several weeks before announcing how sales compared to those of Windows Vista.

So it will likely be some time before more solid numbers are announced. Singling out upgrades complicates things further: People have been registering and qualifying for these reduced-price copies of Windows 8 since June, when new PCs began to ship with the guarantee of a $15 upgrade to the new OS upon launch. Four million may or may not be considered a large number, depending on how it is interpreted in light of total PC sales.

Numbers from Gartner put worldwide PC sales in the third quarter of 2012 (which roughly corresponds to the pre-order and upgrade period for Windows 8) at about 87.5 million. Theoretically, all those PCs qualify for the $15 upgrade. The 4 million represents a 4.5 percent conversion rate for new PCs in the first week — that sounds low, but these numbers are so early that few conclusions can be drawn.

As for the tens of millions of copies shipped to partners, that comes as no surprise. PC makers are banking on the novelty of Windows 8 to sell dozens of new, touch-enabled PCs and tablets, and in order to ship with Windows 8, manufacturers like Lenovo and Acer will have ordered licenses by the million — although they will likely offer Windows 7 for some time to come as a budget option. There are no firm take-aways from Ballmer's numbers, which were likely shared in order to whet the appetite of shareholders, developers and reporters, all eager to know the results of Microsoft's risky experiment in redesigning Windows.

The next few weeks should bring more intelligible numbers and the opportunity to compare them, apples to apples, with Windows 7's sales. Devin Coldewey is a contributing writer for NBC News Digital. His personal website is coldewey.cc.