With not even few moths over after the release of Vista, microsoft has already started its development with Windows Vienna.
Windows "Vienna" (formerly known as Blackcomb) is Microsoft's codename for a future version of Microsoft Windows, originally announced in February 2000, but has since been subject to major delays and rescheduling.

Microsoft doesn't want another repeat of the 5-year gap between the release of Windows XP and Windows Vista, so the company is accelerating its plans for its next generation operating system Vienna.

The time and effort that should have been exerted on Windows Vista were instead diverted to getting Windows XP SP2 out the door and hence the release of Vista was delayed.

Not to forget the release of Vista SP1 is scheduled to release within 6 months.