getting users to logoff

Is there any security threat to the following ?
- windows xp2 is locked (have to enter Ctrl+Alt+Del)
- servers setup to disconnect users at 10:00 PM

For electricity savings reasons we are starting to request users
logoff and shutdown at the end of the day. Right now we have users
who forget to logoff, but their PC will go to the Windows XP locked
screen. If we are in the wrong forum please let us know to repost.
Thanks.
.