Microsoft made waves when it announced that Windows 10 would be available as a free upgrade - but it isn't quite as simple as that.
Here we have explained exactly who can upgrade for free, and under what circumstances you will have to pay for it.
Windows 7, 8 and 8.1 users get a free upgrade - available for one year
If you are already running the Starter, Home Basic, or Premium versions of Windows 7, Windows 8 or 8.1, then you can upgrade to Windows 10 for free.
Users of Windows 7 Professional or Ultimate, and Windows 8.1 Pro, will get upgraded to Windows 10 Pro for free.
Windows Phone 8.1 users will also get a free upgrade to Windows 10 Mobile.
However, anyone running Windows Enterprise and Windows RT will not be eligible for the upgrade.
Those who do update their operating system will get full support from Microsoft until the end of mainstream support in October 2020. The extended support cycle ends in 2025.
The free upgrade is only available for one year after Windows 10 is released on 29 July. Anyone who doesn't update in the next 12 months will likely have to pay the full price - between £80 and £140 depending on which version you opt for.
Windows Vista and XP users, you're out of luck
Over a decade old and no longer a supported platform, Windows XP cannot be upgraded to Windows 10. Users of machines running either XP or Windows Vista must perform a clean install of Windows 10 - and there are no discounts or free offers either; you will have to pay the full price.
Pirates welcome - but you will have to pay
After initially saying anyone running a pirated copy of Windows 7, 8 and 8.1 would get a free, licensed upgrade to Windows 10, Microsoft has backed up slightly. Pirates will still be allowed to update their machines to Windows 10, but they won't get it for free. Instead, Microsoft says it is "planning very attractive Windows 10 upgrade offers for [hardware partners] customers running one of their older devices in a Non-Genuine [pirated] state."
Recognising that some Windows users are running pirated copies without realising it, Microsoft said: "Many customers are unwitting victims of piracy, and with Windows 10, we would like all of our customers to move forward with us together."
Windows 10 for Insiders
Insiders, the name Microsoft gives to its beta testers, will also receive a free upgrade to Windows 10. Already running beta builds of the new software, Insiders will be upgraded to the final RTM (release to manufacturing) copy once it goes on sale.
Updating to this copy will not require a clean install. Insiders running genuine and licensed copied of Windows 7, 8 and 8.1 will be able to get valid Windows 10 licenses and the free upgrade. Windows 10 Insiders who do not have a valid and licensed copy of Windows 7, 8 and 8.1 will have to buy Windows 10.
Microsoft plans to continue the Insider programme, so Windows 10 Insiders will be able to update to new builds as they become available.
Should I upgrade on 29 July?
Microsoft is releasing Windows 10 in 190 countries on 29 July. You can upgrade then if you like, but expect Microsoft's servers to be incredibly busy, likely resulting in slow and possibly even failed downloads. It may be best to wait until the excitement has subsided the following day.
But even then, Windows 10 is unlikely to be perfect right out of the blocks. Beta testers claim even very recent versions of the software contain many bugs and is generally unstable. Many of these problems will have been fixed by the 29th, but we advise against installing Windows 10 on an important work computer for at least a couple of weeks, after which early adopters will have either given the green light, or reported problems which need urgent attention.
IBTimes UK has delved into the top 10 things you need to know about Microsoft's new operating system. Check out our video feature below.