"Windows 8 brings together the best of the PC and the tablet. It works perfect for work and play and it is alive with your world. Every one of our customers will find a PC that they will absolutely love."
Windows 8 will be available for download beginning at 12:01 am local time worldwide and at retail stores.
Microsoft also launched Windows RT, designed for tablets and available pre-installed on new devices including its own Surface tablet, which also goes on sale Friday.
Microsoft reported that pre-sales of Windows 8 have outstripped those of its predecessor by 40 percent.
The software giant gambled by changing long-familiar user interface features to make Windows 8 compatible with trends toward keeping programs and data in the Internet "cloud" and relying on mobile gadgets at work and at home.