Microsoft has announced that its next operating system will beoffered 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."