Those who downloaded many Windows 10 preview editions will have to upgrade by Thursday or face an unbootable PC.
On Thursday, 11 of the past 14 preview builds will time out, Microsoft support engineer RajithR explained in a forum posting. Builds 9926 through to 10166 will go out of service if users haven't upgraded them in the meantime.
"You must take action before October 15, 2015. After this date, you will need to perform a boot from media installation of Windows 10 RTM and opt-in to the Windows Insider Program to continue to get new Preview builds," the Microsoft staffer warned.
"Because your Preview build has already expired, it is no longer activated. As a result, you'll be required to use the Windows 10 default product key to upgrade, and afterwards you will not be activated."
That said, if you're still running a preview build you should have noticed by now - the pre-release operating system has been rebooting every three hours since the first of the month, and warning windows will have been flashing. But if experience is anything to go by, there'll still be some unused review systems sitting in the corner somewhere.
There is still the vast majority of Windows users who Microsoft would also like to upgrade to its latest operating system. At last week's device day, Microsoft pronounced itself well satisfied with the latest upgrade figures but, based on what the spokespeople said, there's still a ways to go.
Microsoft said that 110 million PCs are now running Windows 10 ten weeks after its launch. In addition, eight million businesses are either running or trialing the new operating system at the moment.
That's a fair bump, but well under 10 per cent of Microsoft's user base. Initially you can expect Windows 7 and 8 users to move operating systems first, since it's essentially a free upgrade for the first year, but this may take longer than expected.
Businesses, El Reg suspects, are going to be slower to adopt Windows 10 than Microsoft thinks. Unlike Vista, Windows 7 works perfectly well for most business needs and will do for some years yet, so there's no need to change.
Microsoft can expect more upgrades from consumers, although it nearly managed to snatch defeat from the jaws of victory by making the new operating system too free with its data for many. There was a jump in upgrades after the launch, but the data from online monitors suggest that it's tapering off a little since.
What Microsoft has to crack with Windows 10 is the XP problem. About 12 per cent of Windows users are still running the outdated and unsecured operating system. Upgrading all the way to Windows 10 is going to be an arduous process for them. ®