A few hardy PC users across the UK stayed awake to wait for the updated operating system as the firm staggered its release, allowing some customers who pre-registered to get access before others.
Microsoft is also introducing a new web browser - Edge - to replace Internet Explorer, while the firm’s voice assistant Cortana will also move to desktop computers for the first time.
The much-loved Start menu, which was removed in Windows 8 to much public outcry, is also making a return in the new software, as the firm looks to modernise and appeal to a new range of customers.
Will it cost to upgrade?
Millions of PC users will also be able to upgrade for free as Microsoft is making the update available at no extra cost to those who are already using Windows 7 or 8, though this offer will expire next year.
Those who are not eligible for the free update will be asked to pay £99 for the Home version of Windows 10, which will go on sale at the end of the month.
So should you upgrade?
Windows 10 is Microsoft’s final throw of the dice in attempting to deliver an operating system that will work across all your devices: home computers, laptops, games consoles, tablets and mobile phones.
The company has said there won’t be a successor - just incremental updates once in a while.
In a complete break from tradition, you won’t have to buy Windows 10 - it’s being offered as a free upgrade to users of the previous two versions. But is it any better that its immediate predecessor, the unloved and commercially unsuccessful Windows 8.1?
The short answer is, yes, it’s a little better - but it’s still a close call as to whether it’s worth the time and trouble a Microsoft upgrade traditionally involves, even when it’s free.
The list of new features in Windows 10 lays bare the catch-up game the company is playing here, not to mention its uneasy grasp on popular culture.
Its imitation of Apple’s “personal assistant” Siri, which Microsoft calls Cortana, invites you to invoke its presence by saying, “Hey Cortana” out loud. Can you seriously see yourself doing that in your house?
The return of the Windows Start Menu signals a U-turn on the move to phone-like “tiles” on the desktop, and the introduction of a new web browser to replace Internet Explorer may tempt one or two users away from Google’s Chrome.
There are also new apps for email and photos, but hardly anything that can’t be written off as eye candy. Unless you also use a Microsoft Xbox, which does benefit from some timely integration, your computer will gain little in useful functionality.
Yet the central flaw in Windows 10 is more fundamental still: it is Microsoft’s mistaken belief that a one-size-fits-all operating system across multiple devices is a good idea. It is predicated on the idea that the familiarity of Windows will make unfamiliar new devices more friendly - but we’re all used now to operating our Android and iPhones and tablets, and the changes wrought on Windows 8.1 means it’s no longer familiar anyway. What’s more, the most efficient computers are those running the leanest operating systems; the Windows catch-all model is exactly the opposite.
You will have a year from Wednesday’s launch date to decide if Windows 10 is worth your while; after that, Microsoft says the free upgrade window will close.
If you’re still running Windows XP, the free offer won’t apply and an upgrade will cost £99. Given that you can currently buy a decently-specified complete PC for £150 from dabs.com, that’s clearly a non-starter.
Here’s the bottom line: if at the moment you’re running Windows 7 and you’re happy with it, you will gain nothing substantial by upgrading, Early reports suggest you might even slow down your machine.
If Windows 8 or 8.1 is your current system, by all means go ahead and upgrade - not because the new system is great but because the old one is worse. It’s hardly a ringing endorsement, but then Windows has always been tolerated rather than loved.