Thanks for your reply. I will have a look at Recova. The specific need I have is as follows. My daughter is a lawyer, and her laptop is always full of confidential files. Despite my regular nagging, she has never made the effort to use encryption and does not use a secure delete program when deleting files. (In my experience it is amazing how few lawyers understand the need for this sort of thing). She is upgrading to a new laptop and wants to pass her old one on to a friend. She has been thinking that she will have to have the hard-drive reformatted to securely delete all data, including previously deleted files. My suggestion was to run a program that would overwrite all free space, which would securely delete all previously deleted files and avoid the need for a reformat and reinstallation of the operating system. I used to subscribe to a program called Privacy Guardian, which had this "overwrite free space" function, but I ended up deciding that it was not worth the money in terms of the amount of use I put it to.
The reason I thought this function would go well with CCleaner, is that it is (as its name says) a cleaning program. It is what I use to get rid of everything on my computer that I no longer want. An "overwrite free space" function, is in effect a way of saying "just in case I didn't securely delete everything I should have in the past, I will make doubly sure by overwriting everything that has been previously deleted." However, if Recova can already do this, that is great.