Companies and home users who are getting up to speed on Microsoft's new operating system, Vista, should not expect a pause for breath. Within a year, the name on everyone's lips could be Vienna, not Vista.

Vienna, the code name for the successor to Vista, is likely to be launched in 2009, according to Microsoft's corporate vice president for Windows development, Ben Fathi.

Fathi reportedly told a session at last week's RSA Conference in San Francisco that Vista shipped about two and a half years after Windows XP Service Pack 2, and that Vienna is expected to follow roughly the same time frame.

Vista's launch came five years after the launch of Windows XP. That gap, though,was something of an exception. Microsoft's operating systems have typically shipped more frequently -- every two to three years through the 1990s.

When contacted for comment, Microsoft would not confirm or deny that it is planning to bring out a replacement to Vista around 2009.

"We're going to continue to innovate and deliver great new versions of the OS just like we've done with Windows Vista," a Microsoft spokeswoman said. "We're not currently talking about the next version. We'll be happy to update you when we're ready."

Serious speculation about the likely features that will appear in Vienna will probably begin some time later this year, although Fathi didn't released any details on what may be in it.

Many companies are expected to delay migrating to Vista until at least 2008.

Colin Barker reported for ZDNet UK from London