Hi Luckyb17. Well it could be a lot of things. Note that your OS along with the program files (depending on what you have installed) could take up more than 70GB or even more. I'd suggest that you check the "Downloads" folder, which is located in the User Folder (C:\Users\yourusername\Downloads). It is a default path for your downloads and many programs use it as well. You might have forgotten to delete something from there. You could also try cleaning up temporary files. Here's article which has a few solutions on how to free up storage space and see what's filling up your drive: http://www.howtogeek.com/125923/7-ways-to-free-up-hard-disk-space-on-windows/
Hope that helps. Let me know how everything goes.
Boogieman_WD