Code:
function LastInput: DWord;
var
  LInput: TLastInputInfo;
begin
  LInput.cbSize := SizeOf(TLastInputInfo);
  GetLastInputInfo(LInput);
  Result := GetTickCount - LInput.dwTime;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Label1.Caption := Format('System has been Idle for %d ms', [LastInput]);
end;