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uses [..],ActiveX, ShellAPI, ShlObj, Shfolder;

[..]

function GetSpecialFolder(hWindow: HWND; Folder: Integer): String;
var
  pMalloc: IMalloc;
  pidl: PItemIDList;
  Path: PChar;
begin
  // get IMalloc interface pointer
  if (SHGetMalloc(pMalloc) <> S_OK) then
  begin
    MessageBox(hWindow, 'Couldn''t get pointer to IMalloc interface.',
               'SHGetMalloc(pMalloc)', 16);
    Exit;
  end;

  // retrieve path
  SHGetSpecialFolderLocation(hWindow, Folder, pidl);
  GetMem(Path, MAX_PATH);
  SHGetPathFromIDList(pidl, Path);
  Result := Path;
  FreeMem(Path);

  // free memory allocated by SHGetSpecialFolderLocation
  pMalloc.Free(pidl);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(GetSpecialFolder(Handle, CSIDL_PROGRAM_FILES_COMMON));
end;