After booting up Windows XP and before I’ve logged on, or when I connect via remote desktop to a machine, the machine’s screen saver settings for the Default User will determine what the screen saver is and how it is fired. However, after I log in, the screen saver settings are my personal ones. The settings are stored in the registry under HKEY_CURRENTUSER for me, and HKEY_USER|DEFAULT for the non-logged in user. If no one is logged in, the DEFAULT settings are used.
For example, after a power failure while I’m away from the machine, perhaps I’d like my picture database screen saver to be active
This article How To Change the Windows Logon Screen Saver in Windows XP shows that I need only change the HKEY_USERS\DEFAULT\Control Panel\Desktop registry keys.
See also Screen Saver Tricks and Make an interactive screen saver