PC Storage not adding up?

I have used around 350GB of my 1TB (930GB) storage device. The problem is it says I only have 65GB of space left on the storage and that does not add up. I know there is hidden files and I did all the things possible to clean up files and hidden files on my computer. That includes system files. That only gained me about 5 GB. What is taking up that 550GB? Is there something I do not know about? I am ruing Windows 8.