How can I permanently delete files from windows laptop.?

Question

I am looking to sell my 17inch Sony Vaio laptop, so I decide to reset it using windows and I thought that would be enough to get permanently remove all my files but I have been told that is not so.
My question are:
1. How can I ensure permanent deletion of files so that a buyer can not retrieve my personal files.
2. Can I still apply any permanent deletion process now that I have done the windows reset?

Answer 1

Use Windows installation DVD disc to remove Windows and do complete format entire hard disk at least 3 times

Answer 2

If you want to be absolutely sure, make a recovery media (usb is easiest) then load something like ccleaner and wipe the drive, this writes over any data and then formats the drive, then do a factory reset. This makes it almost impossible to recover any data. If you don't want to do another reset, there is an option to write over only the unallocated parts of the drive, which should be good enough.

Answer 3

format the drive : C