Treesize is a free utility you can download, and it'll show you every last detail about what's on your hard drive. I don't know if you've changed your setup to show hidden files and folders, but that's one possibility. Get Treesize and you'll see for certain.
I have an old IBM desktop which has 140gb hard drive, when I check the properties it shows over 80 used space but I have cleared virtually everything off it. I have used a utility which analyses the disk and that only showed 24 gb worth of data. Any ideas what the rest could be made up of. I am running XP.......thanks