Files not accessible from Finder

I have a strange problem with a used MacBook Pro I am using (early 2011, Mountain Lion)

I have attached images to help you see what the issue is. Basically, there is about 200GB of information that I cannot seem to get to in my hard drive. When I boot up, both in my admin and as a guest, the computer is basically clean: no movies, music, pictures, or extra applications, just the stock set-up. My disk utility, however, is telling me that there are 52 gb of music, 71 gb of movies, etc. I am recieving this computer used from another person, and I am not sure of the steps they took to wipe it before handing it over.

None of these files show up in my finder. I have run the 'show hidden files' command in Terminal, to no avail. When I run OmniDiskHelper, it can't see these 'invisible' files either: it tells me that there is 30gb of info on my disk.

Any suggestions would be welcome. Thanks!