When you build C++ applications using Vista-specific APIs and the Windows SDK for Vista, they won't run on Windows XP. That's because when the executable loads the system DLLs at launch, it looks for those Vista functions which—whether they're actually called or not—are simply not present on Windows XP.
One scenario that uses Vista functions is when you use RegisterApplicationRestart() so your application will pass the Vista Application Certification tests. Another is using the TaskDialog() to display messageboxes that have only OK and Cancel buttons. If you use the old MessageBox() API, the close button on the top right won't look disabled when Aero effects are on. But if you use the TaskDialog, the top right Close button won't display correctly if your messagebox has only OK and Cancel buttons. Of course, you could create your own custom dialog for messageboxes as a workaround, but that's time-consuming.
An alternative solution to the problem is to put the Vista function calls in a DLL. Then you build this DLL using the Vista SDK, but build the rest of your executable and DLLs using the Windows XP SDK. At runtime, you use LoadLibrary() to load the Vista-specific DLL only when you detect that you are running on Windows Vista or later.
Using this technique, your application can run properly on both Windows XP and Windows Vista platforms.