Hi, Gamez,
I think that by erasing and then formatting with the hard disk manufactures, would give you the satisfaction that you need. For eg. Using their tool, be it seagate or WD, and selecting full erase, booting from cd (normally dos programs windows GUI), you could totally erase with Full Erase and the data are completely wiped out . Nothing could be retrieved, as it will format every dba in this manners. Hard disk contains so much dba blocks, and erasing and formatting with their tools is Full Safe method in this regard. It is a clean Slate then. No hidden or nothing could be done with retrievable softwares.
Surfing thro the nets made your porint clear. ? There is no 100% erase of hdd possible. Even shredder softwares usesd hdd could be retrieved. It also says that even replacing with 0s hard disk be retrieved by software worth hundred dollars.
Will it not be better , if you remove the hard disk alone, and sell the computer. You could beat the hdd with hammer to become unusable storage. If you could use , just format and reinstall the windows till it is dead. Dead hdds could be easily retrieved. So, i sincerely advise you to make it not usable by smashing it and also by running some kind of scratchers over it.
Personal informations are valuable than anything esle, is int