Just spent the evening trying to fix the same problem on my laptop. Managed to find this topic on the forum using my sons macbook. Have just managed to download the AVAST update successfully and problem seems to be resolved although running a scan for the 4th time this evening. Wish I had checked for this post earlier -would have saved wasted 2 hours trying to fix. I agree - bet there are loads of AVAST users out there suffering with the same problem.