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That would be a much faster turn-around than Vista, which shipped more than five years after Windows XP, but Vista was exceptional, said Ben Fathi, corporate vice president of development with Microsoft's Windows Core Operating System Division this week at the RSA Conference in San Francisco.

Microsoft originally planned for its XP follow-up to include a number of radical changes to Windows, including a new file system and a reinvented user interface, but after the company's products were hit by widespread worm outbreaks in 2003, Microsoft redirected almost its entire engineering effort to locking down Windows with the XP Service Pack 2 release.

"We put Longhorn on the back burner for awhile," Fathi said. "Then when we came back to it, we realized that there were incremental things that we wanted to do, and significant improvements that we wanted to make in Vista that we couldn't deliver in one release."

Vista shipped about two-and-a-half years after XP SP 2, and Vista's follow-up is expected to take about the same amount of time, according to Fathi. "You can think roughly two, two-and-a-half years is a reasonable time frame that our partners can depend on and can work with," he said. "That's a good timeframe for refresh."

That time line would put Microsoft's next client operating system out by the end of 2009.

Last year, Microsoft said that the code name for this Vista follow-up is Vienna, but Fathi said he could not disclose the current name. "We've been told not to use it publicly," he said.

So what will be the coolest new feature in Vienna?

According to Fathi, that's still being worked out. "We're going to look at a fundamental piece of enabling technology. Maybe its hypervisors, I don't know what it is," he said. "Maybe it's a new user interface paradigm for consumers."

"It's too early for me to talk about it," he added. "But over the next few months I think you're going to start hearing more and more."