What I do if not sure a file or folder is safe to delete is rename them and see what happens.
So for a file called filename.dll I would rename it to filename.dll.7-31-2010-delete
For a folder, rename foldername to foldername-7-31-2010-delete
Note I simply append the date and the word delete (using dashes instead of slashes to avoid directory hassles) so, as typically happens, I forget about it and weeks or months later when I stumble on the file or fold again, I can quickly see how long it has been, and know it is safe to delete or not.
But if I determine I need that file or folder, I don't have to remember what the original name was, I simply rename deleting the extra endings.