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 existing Windows users, as well as those buying new PCs.
Analysts say the company is beginning to move away from relying on Windows as a key money-making product.
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 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.
Mr 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, Mr 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.
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"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." (BBC)