function Is64BitInstallMode: Boolean;
Returns True if Setup or Uninstall is running in 64-bit mode, or False if it is running in 32-bit mode.
When True is returned, it is safe to assume that IsWin64 will also return True.
begin
if Is64BitInstallMode then
MsgBox('Installing in 64-bit mode', mbInformation, MB_OK)
else
MsgBox('Installing in 32-bit mode', mbInformation, MB_OK);
end;