You might try:

Code:
 

run 'RUNDLL32.exe user32.dll, LockWorkStation'



If this is an NT,2K,XP box, This will lock the desktop as soon as it is executed. The problem with this is, how do you decide when to run it, as you obviosly can't just run it everytime someone logs in, or they would have to immediately re-enter their password.
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(... better days ahead)