According to thebetaguy Microsoft is planning a major overhaul for their next version of Windows that should give users more performance and stability. The site claims Windows 7 will be a radical change and won't be binary compatible with previous versions of Windows:
Windows 7 takes a different approach to the componentization and backwards compatibility issues; in short, it doesn't think about them at all. Windows 7 will be a from-the-ground-up packaging of the Windows codebase; partially source, but not binary compatible with previous versions of Windows. Making the break from backwards compatibility is a dangerous proposal but a dream for software developers. Performance of native applications can be increased, distribution sizes can be cut down, functionality can be added without the worry of breaking old applications, and the overall end-user experience can be significantly improved.
However, Windows' lure has always been that applications from older versions of Windows are almost guaranteed to work post-upgrade; this is in contrast to older UNIX solutions where upgrading the system could render old applications useless without access to the source code. On an operating system which uses a binary distribution model, this is an unreasonable expectation. However, there is one company which made a success out of breaking backwards compatibility, using a method which Microsoft are seeking to emulate with the launch of Windows 7. The company in question is Apple.