A recent discussion went on for weeks about this one. They never found it from what I read.
Until you discover the cause just head to the Power control panel and set the display to power down in some number of minutes.
My friend has a pretty new Dell Dimension E520: XP MCE 2005, C2D E6300, 1GB RAM, 160GB HDD, 128MB ATI graphics. Her computer just randomly stopped displaying the screensaver after the set amount of time went by. It doesn't even put the monitor in standby after an hour. Does anyone have an idea as to why it is doing this? If more info is needed, please ask. Thanks.