It's not a question of making Windows see all free disk
space but a question of making you see what's using
up your disk space. The figure of 18 GBytes of used
space is most likely incorrect. These tools might help
you see what's going on:
DriveUse:
http://members.ozemail.com.au/~nulifetv/freezip/freeware/index.html
Bullet Proof Folder sizes:
http://www.foldersizes.com/
You should also run chkdsk.exe /f in order to turn lost
clusters into files which you can then delete.