Yes, I know this thread is old.
For anyone looking for solutions to this problem that did not find the solution above to be helpful, I believe I have a solution for you. The problem as described above IS with the screen saver settings. For some reason Vista is not dumping the MS Vista screen saver settings (for time and password requirements) after they are changed. So if you switch to a screen saver without the options given in the screen saver control panel (such as choosing "(None)") then your computer defaults to the previous screen saver's options.
For example: I had my screen saver set to: Blank, to come on in 5 minutes, and "On resume, display logon screen" was active. It worked correctly. Then I switched to "(None)" for my screen saver which blanked out BOTH options for time and logon screen. Then, like clockwork, every 5 minutes of inactive time I would see the logon screen pop up without a screen saver.
Solution: Switch back to a screen saver (such as Blank). Turn up the time for screen saver to become active (I used 9999). Turn off the check mark beside "On resume, display logon screen". Click "Apply". Then switch back to any screen saver you desire.
You should no longer be plagued by that annoying constant logon screen popping up while you are watching YouTube videos, making dinner while listening to Pandora, or whatever it was that brought you here.