"The survey by IHS iSuppli said the total PC market in 2012 is expected to contract 1.2 percent to 348.7 million units, down from 352.8 million in 2011."
I think it is safe to bet that 90% of PCs sold in 2013 will be sold with Windows 8 installed. 90% of 350M gets you a good way towards that "400M copies sold" figure. Add in some sales for tablets and two months of sales in 2012, and you probably are there already. To get anything different, the economy would have to collapse spectactularly, or people would have to stop buying Windows PCs en masse.
For Windows, the more interesting figures would be how many PCs run Windows 8 (I bet many companies will overwrite Windows 8 with Windows 7 or XP), and how many standalone upgrades got sold.
About 700m units of win7 sold to date. From this win8 is a bit more than 1/25th the sales of win7 So about 28m units sold so far?