ScreenSaver does not activate

The ScreenSaver under windows-XP Family Edition is not activating after the delay setup.
The last time it worked, I think, was just before I installed a Microsoft Wireless Optical Desktop 1000 (Wireless Keyboard and Wireles Mouse). I Used an old mouse (that is better for my hand for its size) I thought it was the problem but when I reconnected the new mouse I had the same problem. Thanks in advance.