Assuming you have already deleted all your documents, pictures etc off the computer, CCleaner can wipe the free space on the hard drive properly. This way the new owner will not be able to restore deleted files with data recovery software. If you tick the right options in the Cleaner section it will also get rid of any saved passwords, website cookies, browsing history etc... from IE, Firefox, Chrome etc..
What make of computer is it? Most branded PCs come with recovery software built in, that restores the hard drive to the way it was when the PC was bought.