Recovering space on C

I downloaded a freeware incinerator and chose to shred the free space on C in
order to get rid of all the files that had been deleted, but were still on
the HD waiting to be overwritten. Since then I show practically no space
left on C, even thou before I used the incinerator only about 1/3 of the
drive was used. Does anyone know what I can do to recover this space?