I don't know if it's still available, but a while ago I was using a program called "hot spots" which I believe was freeware (maybe shareware). I don't know if it is still available or how well it works with WinXP or Vista. It allowed you to move the cursor to a different corner of your video monitor to perform different functions with your screensaver. One was to turn it off. As long as the cursor was there, the screensaver wouldn't start. The problem I had with it was that often a program would cause the cursor to move out of that corner, and then the screensaver would start.
SMK