Ok, you are messing two ‘screensavers’ here:

  1. your own screensaver, the one you usually uses that was already installed and running.
  2. avast screensaver module. It’s not a ‘screen saver’ itself but a module of avast. It scans the system ‘whyle’ your own screensaver (1) is called and run.

The ‘active’ screensaver is yours (1) and you will call it by the avast module (2). If the system freezes, your avast! installation is not good or your own screensaver (1) could be very ‘interactive’ and eat so much system resources or any other incompatibility between (1) and (2).

Hope I can make myself a little bit more clear.