Roughly the same thing has happened to me on each of the last 2 program updates:

  1. Installed the update … OK
  2. Rebooted the PC … OK
  3. Avast! on-access scanner starts up … OK
  4. stays working UNTIL I run the mouse cursor over it. Then it just disappears! Gone. Phut. Not in task manager any more. No messages, no entries in the error log. Nothing.
  5. Just reboot the PC once more and then everything seems to be fine and dandy. On-Access scanner stays alive and does its job.

As I say, exactly the same thing has happed on each of the two latest program updates. Weird.