Rename/Delete Files - Reasons Why It Is Not Possible

This has been bugging me for a while. There are certain files that SOMETIMES can not be renamed or deleted but as soon as you restart your machine, it magically works. One of the logical reasons that I came up with is that they are being used by some processes or something like that... but in this example, I can't see why this .mp3 file that has been sitting on my desktop for months can't be deleted.

I want to know the reasons as to why you can't rename/delete a file. Why is the OS blocking it? Also, is there any software out there that can help with deleting/renaming?