Right click the desktop, choose properties and go to the screensaver setup. Choose Avast as screensaver. Then click on advanched button and set your own screensaver there. That should work.
But there are a few issues witht he Avast screensaver as you can read
HEREIf you leave the comp, why not just use the button on your monitor to shut it down? This will safe more energy than the power saving mode
