First 15 months sales:<p>* Windows 8: 200 million<p>* Windows 7: 300 million<p>* Windows Vista: ~128 million<p>* Windows XP: ~120 million (based on an IDC estimate, as Microsoft released few early sales figures for XP).<p>It looks to me like Windows 7 was the anomaly: a stronger PC market at the time, and 8+ years worth of computers running XP looking for an upgrade. Vista was not a realistic option mainly due to a lack of drivers at release, much higher system requirements, and absurd pricing-- up to $359 for a boxed copy. By the time Windows 7 came out, manufacturers had gotten around to creating the Vista drivers that'd also work in 7, computers that would've been sluggish in Vista could easily run 7, and the price of the OS was slashed.<p>Getting people and businesses to upgrade again when they just did so in the past few years is a tougher sell. The hardware available at the release of Windows 8 was pretty un-exciting too, which probably contributed to 2013's record-breaking decline in PC sales overall. Only in the past 3-4 months have the updated models from most manufacturers made it into stores -- touchscreens, new form factors and doubled battery life -- reasons for people to actually want to buy a new PC.