Deleting Old Files On Hard Drive

I have heard that even though you can delete files from folders etc, there is always a "ghost" copy sitting on the hard drive. Therefore it could be possible, if you were to sell your pc to someone else, that they can retrieve anything that has ever been saved on to the hard disk. So my question is: How do you know if you have removed all your old files from the hard drive, if you can't see them in their folders any more? (Hope this makes sense!) Thanks for any advice given…

data can be recovered from hd, just depends how much trouble one wants to go through to get it. i think deleteing file/folders should be ok as i think most people buying a used computer wouldnt go through the trouble of trying to recover data. there are some wiping utilites, you could formatt the drive, reinstall os etc, for the truly paranoid remove hd, drill afew holes through the platters or grind/sand platters in different directions.

Thank you for that advice.