function GetAppFileName (hWnd: THandle) :string;
var dPID: dWord;
    hHandle: THandle;
begin
  GetWindowThreadProcessId(hWnd, @dPID);
  SetLength (Result, MAX_PATH);
  hHandle := OpenProcess(PROCESS_QUERY_INFORMATION or PROCESS_VM_READ, True, dPID);
  if GetModuleFileNameEx (hHandle,  0 , PChar (Result), MAX_PATH) >  0 
    then SetLength (Result, StrLen (PChar (Result)));
end;

procedure TForm1.FormCreate(Sender: TObject);
var Handle :THandle;
begin
  Handle := FindWindow (nil, 'Microsoft Excel - Книга2');
  Edit1.Text := GetAppFileName (Handle);
end;