One more idea - when you encounter this problem, and while Outlook is still running (i.e. when the file still can't be removed), could you create a dump of the Outlook process (e.g. choosing Create dump file from the context menu on that process in Task Manager) and upload the dump to
ftp://ftp.avast.com/incoming - preferably compressed to a uniquely-named archive, and post the filename here?
It could confirm whether my change is related.
