Interesting, I wasn't able to simulate it on WinXP x64, only ordinary WinXP.
Anyway, the two problems you mention are caused by a Microsoft library (Explorer Extension) and the 3rd party skinning component (the rest of the GUI). So, I'm afraid there isn't much we can do about it right now.
When avast! 5 is released, it will be built by a new version of the compiler, and it won't use skins anymore - so both of the problems should disappear.