21 January 2015 Last updated at 18:13
Microsoft has announced that its next operating system will be offered as a free upgrade to owners of devices running Windows 8, Windows 7 and Windows Phone.
The announcement marks a change in strategy to its previous policy of charging for major updates.
The offer, which is limited to the OS's first year of release, may aid its adoption.
That could help avoid a repeat of the relatively slow uptake of Windows 8.
The event is taking place at the firm's Redmond headquarters.
One analyst suggested the firm needed to change its policy to encourage interest in its ecosystem.
"Overall we know that about only about 10% of computers are running Windows 8 and the adoption rate among companies is similar or lower," said Frank Gillett, an analyst at the Forrester consultancy.
"Developers are not paying much attention to Windows for mass market consumer apps, and you could even argue that for enterprise software most of the energy is going into mobile apps for iPad and Android tablets.
"Windows 10 is in effect a huge invitation to software developers to write exciting, powerful applications that will draw consumers.
"My hunch is that they can succeed in getting a new generation of PC and tablet applications, the challenge is getting people interested in its phones."
Cortana on PCs
Windows 10 brings the same operating system to devices of all sizes, rather than having different ones for PCs/tablets, mobile phones and the firm's Xbox games console.
One of the features highlighted at the presentation was how Cortana - the voice-controlled digital assistant previously limited to Windows Phone handsets - would now work on PCs.
Joe Belfiore demonstrated Cortana, which Microsoft announced it has added to the PC
Microsoft executive Joe Belfiore demonstrated how the software could be told to:
bring up Powerpoint presentation documents
locate photos taken during a specific month
dictate and send emails
tell the user whether or not the weather forecast indicated they should wear a coat later
show where their car was parked
In addition, Mr Belfiore showed how the software could also be used to respond to requests that had been typed rather than spoken into a PC.
Windows 8 had been criticised by some for placing so much focus on touch-centric commands rather than the mouse and keyboard.
But Mr Gillett said he thought the addition of Cortana's voice controls could prove more popular.
"Voice is a going to complement other ways of interacting with the computer not be a substitute," he said.
"If you had to say every command instead of touching or clicking, then that would be annoying.
"But if you can quickly say to Cortana, for example, schedule lunch with my mum next Tuesday, then that is powerful."
Browsing with Spartan
Mr Belfiore also highlighted one of the core benefits of Windows 10: a single app will run on multiple types of device, with the user interface reformatting itself to suit the machine it is running on, rather than having a different program for phones, tablets and PCs.
He showed how this meant more advanced versions of the firm's popular Office programs Excel, Powerpoint and Word could be brought to handsets running Windows 10 than were possible under Windows Phone 8. He also demonstrated a new "universal" Photos app that collates and sorts pictures taken with different devices.
Mr Belfiore also confirmed reports that the firm's web browser, Internet Explorer, was being replaced by a new program codenamed Project Spartan.
This will have Cortana built-in to allow voice commands and provide extra personalised information - such as directions to a restaurant whose website is being looked at.
The software also includes a new "noting mode", which will let users scribble or type over a page and then share it with others.
