I don't think you can do what you're looking to do... a 6000 second delay
is actually an hour and 40 delay. At that point, it doesn't serve much
purpose to either:
a. activate a screensaver
b. lock the computer
We give our users here 15 minutes before the screensaver activates (The
Windows XP screensaver) and the computer locks. This guarantees that the
saver will come on, and also that the computer will lock, affording you a
better level of security (if you should be worried about that... seems that
you are).
You could lock the computer through the screensaver, and I believe there's a
setting you can take to just lock the computer, but not put a fancy graphic
on the screen--just the Windows Locked window, instructing the user to hit
ctrl alt del.
HTH
Ken