To activate and deactivate the screen saver whenever you want, go to the following registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ScreenSavers
To go to this path just press window key + R, a run window will appear. Now type regedit and search for above path.
Now add a new string value and name it Mouse Corners. Edit this new value to -Y-N. Press F5 to refresh the registry. Now you can activate your screen saver by simply placing the mouse cursor at the top right corner of the screen and if you take the mouse to the bottom left corner of the screen, the screen saver will deactivate.
If still it doesn't work, log off or restart your PC.