You need a disk cleanup .... If there are not the backup files, some other junks are there , which is displayed as unknown there. BTW, the cleanup shouldn't consume more place, unless there is a auto backup/restore point creating mechanism with system shutdown.
. Download and install it, and then run it. From the cleaner tab, run the cleaner .... it will remove all the possible junks from your system. After that, dont touch the registry tab ; and come to the tools tab, and select system restore under it. Remove the restore point (the last one is auto disabled, so you cannot delete it; for the system safety).
Now see if you have gained some free space or not .