Clean install not really erasing all old data?

I recently bought a Macbook Pro from a friend who decided to upgrade to the newer macbook pro 17 unibody.

Well he said he'd do a clean install of OS X so that he could give me the laptop clean and with no previous data. Well I got the laptop the next day and I was just messing with it and was reading stuff about the root user, so I decided to try it out and I enabled and logged on as the root user.

To my suprise ALL of his old stuff was still accessible under root user, some programs and his address book and calander events. So Im thinking in my head "I thought he did a clean install". I end up calling him and he doesn't understand it either he said he did a clean install.

Does anyone know why the data is still in there? Even a fresh install of the OSX? Is there a way to totally get rid of it. He also did boot camp for me so I also have windows on there. Is there a way to get rid of the data and re install the OSX without messing the with the windows side of the partition?