apply winexit.scr to all users on a machine in domain environment

Hello,
I'm still trying to figure out how to make winexit.scr be the default
screensaver for all users who login to our lab's machines, which are running
XP Pro. These are public-access computers, users login to them by
authenticating against a domain that I do not control. I also don't control
the users...the University we are a part of has control over the domain.

So what I want is for winexit.scr to be the only screensaver they've got. I
plan to block their ability to change screensavers (or delete the .scr files)
via group policy. I've found an explanation online of how to set winexit.scr
as the default for all users by modifying the registry on a Windows
Server2000 or 2003 environment, just not in XP.

I've also found suggestions for doing this via Group Policies enforced from
the ADUC using GPMC, but so far, we don't do things that way - our group
policies are stored locally on each machine...and I see no way to force the
screen saver that way.

I've tried noodling around in the registry, setting winexit.scr as the
screensaver in HKEY_USERS\Default, but this only effects the pre-windows
screensaver, right before the user logs in.

Does anyone know if what I am trying to do us possible, and if so, how can I
do it?

Many thanks!