Is64BitInstallMode

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;

IsWin64
ProcessorArchitecture