I am using WinMe. He was using Wins Xp. I had a similiar experience with my taskbar icons disappearing also right after downloading Yahoo Instant Messenger program. I was and am using Wins Me and when I would download Yahoo onto my pc from the Yahoo download site, later my taskbar icons would disappear from view. You could take the mouse arrow however, run it from side to side where the icons were and they then would indicate they were there when the mouse was over the top of their position, but I couldn’t see them.
I never could resolve this problem except to uninstall Yahoo Instant Messenger and then that problem immediately stopped. I have another pc that has Wins 98 2nd Edition on it and when Yahoo Instant Messenger was downloaded to it the problem that I had on WinsMe did not occur on the 98 2nd pc.
So, no telling what could have been causing this problem to happen to this individual, but I doubt very seriously that it was caused by having Avast installed on the pc. If I would re-boot my pc usually the icons would appear visible again and then would disappear if and when my mouse was in close proximity to the icons. :wink: