Designed to run on both PCs and tablet computers, Windows 8 heralds the biggest change to the industry's dominant operating system in at least 17 years.
Windows 8 attempts to bridge the gap between personal computers and fast-growing tablets with its touch-enabled interface. The launch event comes amid a rash of other tablet offerings ahead of the holidays.
Apple unveiled its iPad Mini with a 7.9-inch (20cms) screen on Tuesday. Amazon.com and Barnes & Noble, makers of the seven-inch (17.7cms) Kindle Fire and Nook Tablet, are coming out with larger versions next month.
The Windows event in New York also heralds the launch of the software company's Surface tablet, its first venture into making computer devices.
The device goes on sale on Friday, as will computers and other tablets running Windows 8. Wedge Partners analyst Kirk Adams expects Microsoft to trail its rivals in fourth-quarter tablet sales.
One factor that might dampen enthusiasm for the Surface is its price - 599 dollars (£372) with its touch keyboard cover - and its availability for purchase only from a limited number of Microsoft stores and online, Mr Adams said. He said consumers may be reluctant to buy the device until they can try it in person.
Meanwhile, most analysts believe companies and governments will hold off on upgrading to Windows 8 for at least another year. About half of business users still have not upgraded to Windows 7 from Windows XP, which came out in 2001.
XP also marked the last time Microsoft had a huge launch campaign for Windows. Microsoft also came to New York then, holding an event at a Times Square hotel. Releases of Windows Vista and 7 since then have been more subdued.