If you are now using Windows 7 or 8.1 you have until 29 July to upgrade to Windows 10 for free - after that it will be around THB5,000.
The missed target is rough news for the company, which has relied on that promise to attract developers to build apps for Windows 10.
The company unveiled the target when it launched Windows 10 previous year, indicating that it would make the latest Windows operating system by far the most popular the company has ever released.
But Windows Phones running Windows 10 Mobile also were expected to help Microsoft reach that total by mid-2018.Since April 2015, the bottom has fallen out of the Windows Phone market, with Microsoft officials conceding that Windows Phone isn't much of a focus for Microsoft in calendar 2016. Edge, which rolled out with Windows 10, replaces Microsoft's previous browser, Internet Explorer.
Microsoft's reorganization of its phone business, to get away from manufacturing a broad selection of Windows smartphones, has been a key factor in the delay.
Although Windows 10 has received much praise since launching past year, one of the biggest drawbacks is that Windows Media Centre has been revoked - the service that allows users to play DVDs and CDs on their desktop. Since Microsoft share is towards diminishing curve, the company is making several changes in the business model from buying LinkedIn, to improving Skype's telephony service, to building technology around VR. For the mobile Windows OS to bounce back, Microsoft will need to dedicate more resources to it.
A computer screen shows features of the Windows 10 operating system.
Part of this is because of mobile: Windows Phones haven't exactly had a banner year, and they were supposed to be part of this legendary billion.
To get around this, Microsoft has made a decision to include a DVD playback app that can be installed if you have upgraded to Windows 10.
The adoption of earlier versions of the software has been out spaced by the Windows 10 uptake.
This isn't something that's unique to Windows 10, either. But that fervency has clearly cooled off dramatically in the past few months - and will likely do so after this month when Windows 10 upgrades will no longer be free. According to a recent Forrester Research study commissioned by Microsoft, enterprise adopters of Windows 10 are seeing their deployments pay off in just 13 months.