1) If you are using system re-establish, then it could be taking up a bit of space your HDD. If you disable it, it will delete all of old restore points and free up some space. You could then enable it again.
2) There could be some unseen files and folders attractive up some space. You don't want to delete those, though, if window desires them.
3) Try emptying the recycle bin.
4) How much space is allotted to the swap file?