How can I identify what application is using a given file?

Problem

I'm trying to delete some folder but this error is preventing from doing it: Cannot delete [FileName]: It is being used by another person or program. Close any programs that might be using the file and try again. How can I identify what application is using the file? I want to end the application using a Task Manager, but I didn't know what application is using it. The folder is not shared and my user account is an administrator.