[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
#ifdef UNICODE
  #define AW "W"
#else
  #define AW "A"
#endif

const
  MaxStringResourceLength = 65535;
  LOAD_LIBRARY_AS_DATAFILE = 2;

type
  HRSRC = THandle;
  HINSTANCE = THandle;
  HMODULE = HINSTANCE;

function LoadLibraryEx(lpLibFileName: string;
  hFile: THandle; dwFlags: DWORD): HMODULE;
  external 'LoadLibraryEx{#AW}@kernel32.dll stdcall';
function FreeLibrary(hModule: HMODULE): BOOL;
  external 'FreeLibrary@kernel32.dll stdcall';
function LoadString(hInstance: HINSTANCE; uID: UINT;
  lpBuffer: string; nBufferMax: Integer): Integer;
  external 'LoadString{#AW}@user32.dll stdcall';

function TryLoadResourceString(const FileName: string; Ident: UINT; out Value: string): Boolean;
var
  BufLen: Integer;
  Handle: HMODULE;
begin
  Result := False;
  Handle := LoadLibraryEx(FileName, 0, LOAD_LIBRARY_AS_DATAFILE);
  if Handle <> 0 then
  try
    SetLength(Value, MaxStringResourceLength);
    BufLen := LoadString(Handle, Ident, Value, Length(Value));
    SetLength(Value, BufLen);
    Result := BufLen > 0;
  finally
    FreeLibrary(Handle);
  end;
end;

procedure InitializeWizard;
var
  S: string;
begin
  if TryLoadResourceString(ExpandConstant('{srcexe}'), 666, S) then
    MsgBox(S, mbInformation, MB_OK)
  else
    MsgBox('Resource with this ID doesn''t exist!', mbError, MB_OK);
end;