I’ll believe it when I see it, but Microsoft is planning a late 2009 release date for the next version of Windows, codenamed Vienna. That would be just two and a half years after the launch of Vista, which in turn was released more than 5 years after Windows XP.
Many of the upgrades that were planned for Vista had been put on hold when it became clear that the most important item for Microsoft to focus on was security. Therefore Vienna will feature a new file system and a new user interface (no more start menus or toolbars), which had originally been intended for Vista.
Most significantly, Vienna will break backward compatibility with many existing Windows programs. While that will probably make it easier for Microsoft to hit it’s projected release date, I can’t imagine too many folks will be happy about the need to upgrade almost all of their existing software. That said, it’s possible that Vienna could include some sort of virtualization layer that would allow you to run older programs. Heck, if you can run Windows applications on Linux using Wine, why not run Windows programs on Windows?