With this error, it always hard to determine on the Window OS. If this is a test/QA etc.. and if you are able to do so, can you shut down the AE and try to delete that file? That way, if even after doing that, and you still get the same message/error then you can at least verify that something else is still using that file.
The reason I state this, was I had an incident where the file was open by another process, although I had closed all my other open program (some of its process were still running in the background, so it never release it). So in that instance, in my own system, when I shut down the AE and it didn't resolve the issue. I ended up, logging all other user completely off (and myself) and then to log back in. (in other extreme cases, I had to just restart the machine, so that whatever was still using it would shut down).