I've been trying to do the same (i.e. distinguish the sort for folders from files, leaving files sorted by date). A clunky solution for win7 is suggested to me by another superuser post, making use of the 'group by' capabilities. If the file attributes are displayed in explorer (e.g. view > sort by > more... > [check 'attributes']), then you can group by attribute (essentially files vs. folders) using view > group by > attributes, and sort by date modified using view > sort by > date modified. With this, I can get folders to appear at the top, and files underneath this arranged by date.
Why would I want to do this? I'm by no way pedantic in how I organize files, but I do find I end up with a large directory structure - and typically when clicking through this, I'm usually interested in getting to subfolders. When I've reached the folder I want however, I usually will be wanting to access the most recently modified file. So having folders and recently modified files at the top saves scrolling.