You do know by now as what happens when you ‘delete’ a file from your system. It actually does not get erased from it just like that. The only thing that happens is that it gets substituted and then it falls into oblivion. This is what you mean by overwriting hard drive sectors.
Now the thing about Secure file deletion tools is that they actually delete like what you normally do along with replacing the data with junk and hence make them unrecoverable.
Now you might assume that this is safest way to delete a file completely. But you are wrong. Even after you use a tool to delete, there will always be traces of data circling here and there in your system and this will make it impossible to actually delete a stuff from your system completely. These can be in the form of thumbnails, shadow copies ,search indexes and what not.
Now you might be wondering as to if it is well nigh impossible to delete your PC once and for all. This is what you do when you format your disk and then reinstall your Operating system. I know that this does take considerable amount of time and effort from your part but then you have to keep in mind that it does take some amount of care from your part if you want to keep in system safe. Now comes the one very vital question: are the tools for secure file deletion completely useless? Now I can say one thing for sure: they are not perfect but then another thing is that using them would be a tad better than the normal way of deleting but still you can never ensure complete deletion unless you have done a system format and have reinstalled your Operating System once again.