A few weeks ago, my Dell Inspirion 1501 laptop, running Vista, suddenly stopped displaying screen savers for some reason. I tried everything I could Google, including rolling back to a restore point, downloading the new HID driver, playing around with a wireless mouse, etc.
This morning, my wife and I were discussing lithium ion batteries, and I removed mine from my laptop (to check the manufacture date). When I put the battery back in, the screen saver magically started working again. Go figure. Something to try if you're having the same problem...