Take Over the Screen Saver
To activate and deactivate the screen saver whenever you want, goto the following registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ScreenSavers. 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 screensaver 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 screensaver will deactivate.