Yeah,thats the main reason to use heuristics now. avast! is not as known as Symantec or McAfee,so there would be a very small chance that virii writers will fine tune it against avast!. Maybe more flexible Blocker could do half of this job,but in current form it simply fails to do anything. And finetuning virii to avoid heuristics is a time consuming thing. Source code for them is not available so you have to test every and each modification. And there is no 100% success rate in this.