Few days ago, David J Taylor started a borland.public.delphi.language.delphi.win32 thread on how to disable Aero interface on Vista programmatically.
Yesterday, he found and published an answer. David, thanks!
I know many are still struggling with old applications and their compatibility with Aero. Maybe this will help.
I have repacked David's function into two functions in the DSiWin32 library (DSiAeroEnable, DSiAeroDisable) and added a third one that checks whether Aero interface is enabled (DSiAeroIsEnabled).