Then it is likely that you have a shell extension (context menu item) clash.
avast has to hook files to scan them before allowing them to run to be able to protect you. Unfortunately there are other such shell extensions that can do the same and some of them aren't even security applications.
I had this happen with XP Pro SP3 32bit, I had had avast for some time but after updating a program the problem started. I had been investigating for a long time trying to find why this was happening to no avail, until I came across a topic that had the same problem with SnagIt image capture, which I use.
I used Shell Explorer (shexview.exe) to view what shell extensions were present, I found the one for SnagIt and disabled and no more problems. I had used this Shel Explorer before looking at more security based extensions and never dreamt it would be my image capture software, but the earlier version of it never had this problem, but 9.1 did.
So I don't know if this shell explorer tool will work on 64bit, but perhaps you can find a way and investigate what shell/context menu extensions you have running.
