Navigate to the following registry key:
HKEY_USERS\DEFAULT\Control Panel\Desktop
In the details pane (right-side), double-click the
SCRNSAVE.EXE string value item.
In Value data , type the path and name of the screen
saver, and then click OK .
IMPORTANT : Make sure that you specify the path correctly
to the screen saver. If the screen saver is located
in %SystemRoot%\System32, the explicit path is not
required.
Change the Logon Screen Saver 
