I haven’t fully tested this for Vista yet but it has the same key as XP so I am assuming it works the same. This is very similar to the setting up a default wallpaper but you use the SCRNSAVE.EXE key instead of Wallpaper.

Browse the registry to

HKUS.DEFAULTControl PanelDesktop

Just change the SCRNSAVE.EXE key to wherever your .scr file is located. There is also options for setting if there is any screensaver, ScreenSaveActive, the time out, ScreenSaveTimeOut, and if the screensaver is secure which seems dumb to me since no one has logged into the computer yet, but whatever.