Windows Login screen saver doesn’t run

We’re using a pc with windows XP Pro acting as our office server. We have a few users, each with their own User name. We have the User login screen come up and waits for a login. At night we log out and it sits at the User login screen. We do not turn off the machine. The problem is that for the last week or so the screen saver does not come on anymore. This is not good when it stays on all weekend. I’ve tried right-clicking that screen but that doesn’t do anything.
Any advice? Thanks.