CCleaner has a component called Drive Wiper that will scrub the empty part of the disk (that with nothing on it) but that leaves your current software untouched.
CCleaner can also clear out temp files and so on, but may not clar out all personal informaiton.
but if you have got some very important and private stuff on your hard drive the only complete secure way is too take it out and smash with with a lump hammer, sell it without a hard drive ,you can buy new ones very cheap