I would suggest there are 6 "levels" of deletion:
1. You delete the file from the folder but it's still in the recycle bin, these can be recovered by most people within seconds.
2. You delete the file from the folder and then from the recycle bin, there is a high chance that it can be found using commerically and freely available data recovery programs if someone decides to try and look for it within a few days of deletion.
3. You securely delete the file with a secure deletion program, this should have overwritten it's "space" on your hard drive with random junk or all 1s or all 0s, the file is effectively irrecoverable BUT temporary copies of it may still be recoverable using data recovery programs to search through free space of the drive. Temporary copies are made when files are copied between locations, written to CD s or DVD s, extracted from zip archives. There is also a chnace that older versions of the deleted file migth be lying around on the hard drive.
4. You securely delete the file and wipe free space, or do 2 and then wipe free space. The file is efectively gone forever, as are any temporary or older copies. There is a chance that the most advanced data recovery programs with the most skilled of users could recover these, perhaps from copies or referencs to them stored in files of the operating system, but it's only a chance.
5. You deete the file, wipe free space, the fully erase everything on the hard drive, before finally reinstalling the operating system. The file is effectively gone forever, there is a small chance of there being something left, but this is good enough for all but the most damaging of secrets being hunted by the kost determeined of snoopers.
6. You securely delete the data, wipe the free space on the drive, the wipe everything else on the drive, then physically destroy it and ensure the pieces of it (and probably of the computer it was in) are all ruined far beyond ever being useful again, burn them with fire, drown them in water, expose them to the strong magnetic fields (like around an NMR/MRI machine). No-one, no corporation, no nation, would ever be able to recover data disposed of like this, nless of course there were remnants left on another device which wasn't destroyed, or it had been captured and copied by an eavesdropper if it was ever transferred between computers.
What you do for data depends on the secrecy of the data, and how likely you think people are to be trying to find it.
If you're not even sure you want it gone (1) is enough, but that won't give you the extra free space that (2) would.
If it isn't a secret that needs to be deeply secret then (2) is enough, as long as no-one is looking for it.
If it's something fairly secret (like any of your financial data) then (3) should be done, and atleast (4) should be done before thinking of selling or throwing away the machine. For routine preventions against loss of theft of a device (3) is often enough, unless it's had a lot of financial info on it, as a note it's a good idea to atleast keep all your fincancial stuff within an encrypted 7z archive ( http://www.bleepingcomputer.com/download/7-zip-for-windows/
). Having these encrypted archives means any thief would be unable to get at the main copies of secret data, he could try and find deleted copies and temporary copies but (3) is enough for normal protection.
If you are getting rid of a computer (4) should be done if it's ever had anything reasonably private on it, maybe even (5) if you think there is something really personal which might have somehoe been stored in other places than the file.
(6) is what militaries and governments do when they've finished with computers in their more sensitive branches, if something on your computer is secret enough that it needs (6) then you have a problem because it might be stored in other places too, like yout ISP's records, or somehow it accidentaly got onto a removbale device.
Edited by rp88, 31 March 2015 - 06:20 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB