but... why are you still using 16-bit then?
64-bit programs are actually the fastest... for reasons that should be obvious... but they (again, obviously) won't run on 32-bit programs.
On the other hand, windows.h does define WIN64 or another similar constant, so it's not too difficult to have a 16-bit,32-bit, and 64-bit version of a program from the same codebase.