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

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

// The GetLastInputInfo function retrieves the time
// of the last input event.
// Minimum operating systems: Windows 2000