Just to throw this out. I did experience similar behaviour with a couple of previous versions. I can't recall which ones. I'm not one for using each version as it was released. I just wait to be "forced".
What I do remember, it's been about a year since I saw what is being described.. It wasn't just during a full scan, but if the computer had been used for a log period of time. This didn't happen each time. It was probably a combination of programs previously opened, avast and windows poor resource mangment.
As mentioned, I haven't seen that happen lately. Perhaps something in avast has changed. This computer or OS certainly hasn't.
Just to add a little more. I used to use mcafee with 64mb ram, no icon problem. Even with their infamous 4400 engine and current ram, there was no problem. Only with avast and current ram did I ever experience it.
I actually found it amusing, that I could erase the icon by dragging the mouse across it. The button was there just no icon on it