Click the start menu, run, and type gpedit.msc press return. and group policy box will appear. There is an option to disable the screen save changing in windows.
It's in User Configuration/Administrative Templates/Control Panel/Display.
The option you want is Screen save executable name, specify the screen saver you want there and no one will be able to change it..
Click on the 'Explain' Tab and it will tell you how to use this feature..
Hope it helps.