[NumLock]

Код:

type
  TForm1 = class(TForm)
    ComboBox1: TComboBox;
    Memo1: TMemo;
    procedure ComboBox1Select(Sender: TObject);
    procedure ComboBox1KeyPress(Sender: TObject; var Key: Char);
  private
    { Private declarations }
    procedure SetDef(var Msg: TMessage); message WM_USER+$1001;
  public
    { Public declarations }
  end;
...
procedure TForm1.SetDef(var Msg: TMessage);
begin
  ComboBox1.Text:='d:\Documents';
  Memo1.Lines.Add(ComboBox1.Text);
end;

procedure TForm1.ComboBox1Select(Sender: TObject);
begin
  if not DirectoryExists(ComboBox1.Text) then
    PostMessage(Handle, WM_USER+$1001, 0, 0)
  else
    Memo1.Lines.Add(ComboBox1.Text);
end;

procedure TForm1.ComboBox1KeyPress(Sender: TObject; var Key: Char);
begin
  if Key=#13 then ComboBox1Select(ComboBox1);
end;

оно?

http://zalil.ru/33615616