Screensavers are a common source of viruses, it could be that yours was infected and Norton has removed it. Try going into display options and see if it lets you reselect it but I imagine Norton has removed it from the system.
But as Allan said, screensavers aren't needed nowadays with LCD monitors, you would be much better off setting your PC to turn off the display after some time and even automatically sleep as this would save power, leaving the monitor on and the computer sitting processing all the fancy graphics in your screensaver only uses up unneeded power.
