CCleaner has a "free space" cleaner, plus much, much more. You can clean as you delete your files, overwriting them up to 35 times, if you want to take it that far. But keep one thing in mind, with the "free space" overwriters, if you clean 100% of you free space, you may overwrite all of your restore points. You certainly don't want that to happen.
You have another option, which actually recovers your deleted files, so that you may choose to recover or delete the found files. A two in one deal. The program name is Recuva, written by the same group as CCleaner. What you do is a "deep scan", which may take a while. Afterwards, you have the option of recovery or deletion of the files. They also can be overwritten up to 35 times, but on a large drive, this will take some time. You can actually use your computer while the files are being overwritten.