San Francisco, November 2:

Microsoft Corp on Tuesday announced online versions of its Windows operating system and other popular software programs, hoping to defuse a growing threat from Google Inc. and other fast-moving challengers.

With a new Web site called “Windows Live,” Microsoft hopes to create a new platform that will unfasten some of its applications from a computer hard drive.

The change reflects Microsoft’s recognition of the growing demand for applications and services that can be used from any place, at any time, as the lines between the home and office blur and portable computing devices become more powerful.

“It’s a revolution in how we think about software,” Microsoft Chairman Bill Gates told reporters and industry analysts Tuesday.

“This is a big change for...every part of the ecosystem.” In many cases, Windows Live - available at Live.com - will offer souped-up versions of services like online mapping and instant messaging that have long been available on Microsoft’s MSN.com, a heavily trafficked site that will continue to operate.

Gates emphasized that neither Windows Live nor another service called Windows Office will replace the operating system or other popular applications, such as word processing and spreadsheets, sold on disks that are installed on individual hard drives.