Windows Vista (nee Longhorn) was always supposed to be compatible with Win32 applications, but folks have a natural tendency to worry. Microsoft’s Nick Kramer sets their minds at ease:
Can you please explain the relationship between Win32 subsystem and WinFX. In PDC it was told that WinFx replaces Win32. Now it looks like USER and Win32K are still in use
Good question. WinFX is where all the innovation is, both in the Longhorn/Avalon V1 timeframe, and in releases beyond that. But at the same time, we can’t break existing Win32 applications. So in the Longhorn timeframe, the best way to achieve that was to build Avalon on top of (a subset of) Win32k.sys.
At the end of the day, our goal is not about creating Avalon or replacing Win32k.sys, it’s about getting innovation into the Windows platform. If we could do that by adding features to Win32k.sys, we would. But the compatibility burden for Win32k.sys is incredibly high, so there’s no way we can innovate there in anything like the way we can with Avalon. And in Longhorn, we found we could bring plenty of innovation without completely replacing Win32k.sys.
Practically, Win32 compatibility was never in doubt for obvious reasons. That doesn’t mean that some Vista tweaks won’t be required, just like for any new OS version.