CreateProcess call returns error code 50

Try to use Createprocess to start windows Explorer in a given path, but I keep getting

System Error. Code 50. The request is not supported.

What am i doing wrong?

procedure TfrmProjectManager.OpenFolderinExplorer(const aPath: string);
  function GetWinDir: String;
  var
    Buffer: array[0..MAX_PATH] of Char;
  begin
    GetWindowsDirectory(Buffer, SizeOf(Buffer));
    SetString(Result, Buffer, StrLen(Buffer));
  end;

var
  strCmdLine    : String;
  fStartInfo    : TStartupInfo;
  fProcessInfo  : TProcessInformation;

begin

  try
    if sysutils.DirectoryExists(aPath) or 
       (MessageDlg('Folder [%s] not found. Create it?', mtConfirmation, mbYesNo, 0)=mrYes) then
    begin
      sysutils.ForceDirectories(aPath);
      FillChar(fStartInfo,sizeof(fStartInfo),0);
      FillChar(fPRocessInfo, Sizeof(fProcessInfo),0);

      fStartInfo.cb:=sizeof(fStartInfo);
      fStartInfo.lpReserved := nil;
      fStartInfo.lpDesktop := nil;
      fStartInfo.lpTitle := nil;
      fStartInfo.dwFlags := STARTF_USESHOWWINDOW ;
      fStartInfo.wShowWindow := SW_SHOW;
      fStartInfo.cbReserved2 := 0;
      fStartInfo.lpReserved2 := nil;

      strCmdLine := '"' + GetWinDir + '\explorer.exe"';

    if not CreateProcess(nil,PChar(strCmdLine),nil,nil,False, 0,nil,PChar(aPath),fStartInfo,fProcessInfo) then
     RaiseLastOSError;

  end

  except
    on E:TObject do
      if not IsAbortException(E) then
        raise;
  end;
end;

I tried various combinations of parameters in CreateProcess, but just don't seem able to find the correct one.

I'd say that you shouldn't be using CreateProcess here. Rather than debugging your CreateProcess I'll offer you what I believe to be the right way to open a shell view onto a folder. Call ShellExecute.

ShellExecute(0, '', PChar(aPath), '', '', SW_SHOWNORMAL);

This way you let the shell decide on the appropriate way to display the folders contents to the user.