Peter, please take a look at the attachment, I hope it helps.
You have to go into AutoCAD options (either right click on blank space in a [new] file or via command "_options"), then it should be the first tab (files [hopefully, I don't work with the english version]).
I think you could browse through the tree and mark any directory path and click "browse" on the right side so that you get a dialog that opens a tree view (not the windows explorer dialog for files, but only for directories).
Or you can follow the exact way I highlighted in the picture (though it's originally in german language).
However if you browse a file dialog before the directory dialog (like browsing for the new file template instead of browsing the template directory) AutoCAD will freeze.
One thing is interesting and maybe a useful information for Avast:
On my home PC I have AutoCAD 2015 64bit - Windows 7 x64 Home Premium and there this issue does not appear.
(For reference [as written in the other thread]: At work [where I have this issue] is AutoCAD (Mechanical) 2010 64 bit. As it's a complete suite Inventor 2010 is running, too but that does not get affected.)