I\'m developing a C# .NET business application that needs to work on Windows 7, Windows 8 and Windows 8 Tablet.
Windows 8 will still have win32, so all your applications which are win32 based example winform, C++, COM application will still run fine without recompiling. However Win32 will not be present on surface tablets, so they would not run there. You will have to recode your application using metro style which will intern use WinRT (windows runtime).
For details refer to my blog on windows 8