Some 14 million people installed the Windows 10 operating system in the first 24 hours following its release, Microsoft said, calling the response "overwhelmingly positive."
The company said its new operating system aimed at computers, mobile devices and other gadgets got off to a strong start toward its goal of reaching one billion devices.
"We're humbled and grateful to see the response to Windows 10," corporate vice president Yusuf Mehdi said in a blog post late Thursday.
"We have seen unprecedented demand for Windows 10, with reviews and customer feedback overwhelmingly positive around the globe."
The stakes are high for Microsoft as it pushes out the new operating system for both traditional computers and mobile devices such as tablets and smartphones.
The company is hoping the new system can help it gain traction in mobile, where it lags behind Google Android and Apple iOS, and in emerging technologies for computing.
Windows 10 -- Microsoft skipped directly from Windows 8, which got a lackluster response -- is being offered as a free upgrade for most devices, making it possible to be available quickly on billions of devices.
It will allow for voice, pen and gesture input, and in some cases biometric identification for improved security.
Mehdi said the company is rolling out the software in phases to make the transition easier.
"Our top priority has been ensuring that everyone has a great upgrade experience, so, we are carefully rolling out Windows 10 in phases, delivering Windows 10 first to our Windows Insiders," he said.
"While we now have more than 14 million devices running Windows 10, we still have many more upgrades to go before we catch up to each of you that reserved your upgrade."
In one sour note, the chief executive of Mozilla, which makes the Firefox Web browser, complained that Windows 10 imposes the new Edge browser as the default option, overriding choices made by users.
"The update experience appears to have been designed to throw away the choice your customers have made about the Internet experience they want, and replace it with the Internet experience Microsoft wants them to have," Mozilla CEO Chris Beard said in an open letter to his Microsoft counterpart Satya Nadella.
Beard said the new operating system makes it more complicated to choose a competing browser such as Firefox.
"It now takes more than twice the number of mouse clicks, scrolling through content and some technical sophistication for people to reassert the choices they had previously made in earlier versions of Windows," Beard said.
"It's confusing, hard to navigate and easy to get lost."
Beard urged Microsoft to "respect people's right to choice and control of their online experience by making it easier, more obvious and intuitive for people to maintain the choices they have already made through the upgrade experience."