Windows 10 marks a "new era" for personal computing, Microsoft's chief executive has said.
The software, launched globally on Wednesday, is the company's attempt to reverse its fortunes in the mobile industry.
Windows 10 will be offered as a free upgrade to most consumers.
However, companies will have to pay for their version, as will PC-makers to pre-install it. Analysts say the strategy is designed to speed adoption.
Speaking exclusively to BBC News, Satya Nadella said: "Windows 10 is a huge milestone for us as a company, and quite frankly the industry."
Microsoft is staggering the release over several weeks, so not everyone will be able to get the upgrade on the launch day.
Microsoft has until now released a new version of Windows every few years.
Windows 10 will be the last launch of this kind, the company said - from here on it will gradually update the software for free over months and years.
Nadella said he hoped features like digital personal assistant Cortana - comparable to Apple's Siri, and Google Now - would set Windows 10 apart.
"I'm really excited about Cortana," he said.
"I think of it as [being] as profound as perhaps the PC operating system.
"If you think about our history in technology, we've had concepts that have changed how people have interacted with their computing resources.
"One of them was a graphical user interface, the second was the browser and the web. I think of Cortana as the third platform."
Speaking about possible privacy concerns, Nadella took aim at companies like Google who use data to sell advertising.
"One of the foundational pieces of making anything more personal is trust," he said.
"We're not trying to sell you advertising, we're trying to in fact sell you software or devices so you as a user can trust it, that it's working on your behalf.
"I as a consumer may want to sometimes trade off my data to get a free service, and that's ok. But it's the other users of that same data - that is where trust matters.
"I absolutely want Microsoft to be trustworthy. How consumers make choices between companies, I'll leave it to them."
Nadella argued that future devices, like augmented reality headset Hololens, would maintain the relevance of Windows 10.
The developer version of Hololens, which is placed over a user's eyes and displays graphics in their surroundings, is on course to be released within a year, Nadella said.
Consumer versions of the software would follow at a later date.
But Microsoft's poor track record in mobile means Nadella has had to change the firm's focus.
He has ramped up the company's efforts in creating apps for rival systems - Apple's iOS and Google's Android.
Nadella told the BBC this wasn't simply "plan B" after falling so far behind in the sector.
"I don't think so. I think this is Plan A to me.
"I don't think of this as some zero-sum construct where our presence on other platforms is taking away from the Windows ecosystem.
"If anything, if you're using a lot of Microsoft applications on these other platforms, you might in fact find any one of our devices pretty attractive in your life."
Analysts still see Microsoft's small presence in the mobile world as a serious Achilles heel.
"Consumers are spending the most time on smartphone," said Geoff Blaber, a Silicon Valley-based analyst for CCS Insight.
"This is Microsoft's big, big challenge, because they're only on 3% of smartphones.
"They need to be engaging people on the mobile screen to ensure that their surfaces are being used as far and as widely as possible."
While the company is still to make a small number of mobile devices - a couple of smartphones, and its Surface tablet-laptop hybrid - it has significantly lowered its ambitions in the area.
Earlier this month, Microsoft wrote off last year's $7.5bn (£4.5bn) acquisition of Finnish mobile giant Nokia, while laying off thousands of the employees it gained from the phone company.
The write down resulted in the company's biggest ever quarterly loss.
The deal was made by Nadella's predecessor, Steve Ballmer.
But Nadella refused to call the Nokia move a mistake.
"That's not how to look at it," he said.
"When you acquire and you bring large organisations together there is a certain amount of unfortunate rationalisation that happens."
Windows 10 is muliti-platform - meaning essentially the same software can power PCs, smartphones, tablets, games consoles and wearables.
Nadella hopes this will prove an incentive for developers to design software for the system.
The previous version of the operating system - Windows 8 - was so badly received the company leapfrogged making Windows 9 altogether in an attempt to distance itself from its previous outlook.
In the run up to launching Windows 10, Microsoft launched its Insider Program - a scheme which allowed millions of users to test out new features and provide feedback which was monitored and acted upon by the team.
Initial reviews of the software have been positive.
Nadella said: "We want Windows to go from where users need it, to choose it... to loving it."