What you want is quite possible, but all installers I ever saw either comes in two (or three) different parts: for WIN32, x86-64 and Itanium IA-64 architectures, see
http://en.wikipedia.org/wiki/X86-64[
^] and
http://en.wikipedia.org/wiki/Itanium[
^]. Those installers packed in one file still give the user a choice of target architecture, for a good reason.
Why not giving the user of 64-bit system and option to install 32-bit version? It can run under WoW64 (
http://en.wikipedia.org/wiki/WOW64[
^]) which is a valid choice.
—SA