procedure TForm1.FormCreate(Sender: TObject);
var
  i: integer;
  OldErrorMode: Word;
  OldDirectory: string;
begin
  OldErrorMode := SetErrorMode(SEM_NOOPENFILEERRORBOX);
  GetDir(0, OldDirectory);
  i := 0;
  while i <= DriveComboBox1.Items.Count - 1 do
  begin
    {$I-}
    ChDir(DriveComboBox1.Items[i][1] + ':\');
    {$I+}
    if IoResult 0 then
      DriveComboBox1.Items.Delete(i)
    else
      inc(i);
  end;
  ChDir(OldDirectory);
  SetErrorMode(OldErrorMode);
end;