Same here with win xp. I've tried EVERYTHING, clean install/uninstall, etc. This only happens with avast. Start a scan and instantly, cpu goes to 100% and the mouse is intermittent. After running the scan for a while, cpu goes to normal and the mouse works and everything is ok. The icon does not revolve during the 100% usage.
If this is a bug, I wish avast would admit it or offer a solution.