San Francisco: Microsoft's new Windows 10 operating system debuts tomorrow, as the longtime leader in PC software struggles to carve out a new role in a world where people increasingly rely on smartphones, tablets and information stored online.

No one's expected to line up overnight for Windows 10, the way people did 20 years ago for Windows 95. But Microsoft is counting on tens or even hundreds of millions of people to download its latest release for free in the coming months.