Click Macintosh HD in your sidebar. See what it has for recognized capacity as opposed to just "available". If your finder was on another disk or mounted filesystem, that would explain the discrepancy. Otherwise I would start at that root folder to see where some of the data is being kept. If it doesn't show up in one of the folders shown by default, it may be in a hidden one. At that point trying to figure out what is tying up the disk becomes a greater issue. In the end if you can't figure it out and don't want to take it to Apple, you could try reinstalling just the OS to see if that solves your problems. I would ensure you have your documents backed up prior to doing that.