Why does my harddrive say full?

OK I am very literate with computers so if the answer is advanced, let it be so.... I just did a fresh install of Win 7 Home on my 320gig HD and the HD is saying full when it clearly isn't I have compressed the hard drive files, defraged, and even looking for a data trail to see if some of the stuff from my previous files where still intact. I could not find anything... HELP!!!!