Every get one of those frustrating errors "Cannot access file because it is being used by another process"? For example, you may be trying to delete or rename a file, but this error message won't let you. Sometimes it is as simple as closing an application that obviously has locked that file, but sometimes it's just more complicated - perhaps some service (which has no GUI, and therefore may be less intuitive) has locked the file. Ideally you could just click the file in Windows Explorer and it would tell you what locked the file and what to do.
There is a solution - Sysinternals provides a free tool, Process Explorer, that lets you find all the processes accessing a file. The tools requires no MSI; you can directly copy it to your machine and run it. It's like Task Manager on steroids (in a good sense).
Initially one may make sure that they don't have the file open in some sort of editor, but the problem may persist even if everything is closed. Go to Find > Find Handle, and type in the name or directory of the file. Click search, and it will show you all processes accessing that file. You can now kill those processes such that you can delete the file.
This is actually very educational - you'll see that IIS, other instances of Windows Explorer, certain background services, or apps that you wouldn't think of, all can lock files.