I said some of your files were actually yours. If you have a 730GB disk and you've only ~40GB left, you've got to have something stored to take up all that space - windows only takes around 6GB. If you're looking to delete files that you don't know what they are, you're looking in the wrong place. You should be looking through files you know what they are, as you created them. Don't just go on deleting sprees.
Anyway, if you try to delete a critical system file, windows will stop you.
Anywhoo, you've got a ton of files - music, pictures, video, documents, something - that are taking up your space. Either move those to a different disk or delete the ones you don't need. You really shouldn't be expecting windows to lose weight for you.