good question. you're probably right about being able to modify the registry. I believe you have to be in the administrator group, so, like you said, you wouldn't need to do this in the first place. I guess the benefit would be that you could modify the screensaver to let you on whenever you wanted (after keystrokes were pressed, or something similar). that way, you might be able to log in without using your account. so, this still would be beneficial if you had access to an administrative account (shouldn't be hard) for a short time.
I'll help verify that it'll work in the first place.
One of the things we used to do to get passwords is make a fake login screen.
you can figure out the rest