The software, formerly known under its codename Longhorn, is initially being made available to a select group of 10,000 technical testers via Microsoft's technical beta programme, a company spokeswoman told vnunet.com.
The Vista beta has been offered to developers and corporate IT departments to allow them to test how the software interacts with other systems and applications inside their organisation.
Although the test version does not yet include the final graphics and other features that will be most visible for end users, it does offer a view of new technologies under the hood.
Windows Vista is scheduled for release late next year. A second beta is expected sometime in early 2006.
Microsoft has also released test versions of Internet Explorer 7 and an early beta of Longhorn Server, the next-generation server operating system that will replace Windows Server 2003.
Organisations intending to switch from Windows XP to Vista should limit their testing to the software's APIs, and not attempt to test new features or their functions, according to Michael Silver, research vice president with Gartner Research.
"You should wait at least until beta 2, if not longer, before beginning testing in earnest," he said.
Companies planning to jump from older versions of Windows directly to Vista have less time, Silver warned.
Support from independent software vendors for older operating systems is likely to start waning by 2007, and Microsoft is scheduled to stop all bug fix support for Windows 2000 by mid-2010.
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