|(1) If you're using system restore, then it could be taking up a bit of space on your HDD. If you disable it, it will delete all of the old restore points and free up some space. You could then enable it again.|
(2) There could be some hidden files and folders taking up some space. You don't want to delete those, though, if Windows needs them.
(3) Try emptying the Recycle Bin.
(4) How much space is allotted to the swap file ?