Cannot delete the file from the itunes folder

I cannont delete the file from the itunes folder because it tells me I need permission to do so. I am the admin person for the laptop plus I have checked my permissions under the security tab in the files properties. How do I delete this file to start again? It downloaded 1.2GB of the file before the error message appeared, so its paid for and all but not looking like I'll get to watch it before the time runs out on the movie being available for me to watch. :-(