Let’s imagine you need to turn over your old computer to friends or family, you for some reason do not wish to re-install Windows all over – well there is a middelground that I imagine could be used in case it’s close friends or relatives. Remove all your personal stuff, documents, mails etc. from the computer, remember to empty the recycle bin, clear all browser caches and clear restore points – if possible create a new user and from this delete your old user profile. Final step is to run the command below, this will wipe all free space on the disk – the command is a buildin Windows command that was introduced back in WinXP, so no need for additional software etc. Is it safe enough? Well as I say, if it is close relatives or friends it may be ok as long as you are sure that all sensetive data is removed, but I would likely not advice this for a computer you sell etc. Again, it all depends.
Command to issue;
(for the C: drive, replace C: with other drivelettes as you need).