|If you are truly concerned about the files overwriting them only once will not totally erase them. Even writing ones and zeros to the entire drive multiple times will not fully assure that some data might be recoverable.|
This advice is coming too late for the drive in question but I would advise you to avoid writing any information you wouldn't want someone to have access to on the operating system hard drive. You can use external drives or optical disks for that purpose. CDR & DVDR are cheap and secure.
The only sure way to fully protect from someone gaining access to the data is to remove the hard drive from the computer.
Of course then the computer will have much less value, if your intent is to sell it.
If the computer is an OEM that came with some version of Windows pre-installed AND you have the restore disks for it, then you can use a program like kill disk to wipe the drive and feel fairly confident that short of some expensive recovery efforts the data will be gone.
Then use the factory restore disks to reinstall to factory state.
You can download kill disk from the link below. Be sure to unplug any external drives or media prior to running kill disk.
The free version only makes one pass. You should be able to run multiple times if you desire. Not sure about that. One pass is good enough to prevent the casual user from recovering any useful data.