How To Completely Remove Data From Hard Disk Drive In Windows 7
Many Windows users assume that, as soon as they empty the recycle bin, deleted data contained within it is gone for good, without any possibility of recovering it. In reality, however, this is far from the truth. What actually happens is that the space on the disk previously occupied by the deleted files is given over to free space. When you copy or create new files to the hard drive, or you install a new program, the free space will eventually be overwritten, either in whole or in part. Until then, however, the data remains intact.
If you are planning to sell your computer or hard drive, or you want to securely and permanently delete sensitive data for any other reason, then you'll need a third-party tool to do this The more you use the hard drive, the less likely it is that the data will be recoverable, but you cannot rely on this if you want to be sure that the data is removed. Tools which allow you to permanently delete a file and all of the data contained within it include Recuva, SDelete and Eraser amongst others.
Permanent data deletion is quite different to simply moving a file to the Recycle Bin and then emptying it. Instead of the file just disappearing from Windows Explorer and being marked by the file system as "deleted," a file needs to be written over entirely for it to become completely unrecoverable. Data deletion tools overwrite the space with zeros, removing the old data entirely. If this is not done, then someone may be able to recover the data by scanning the hard disk with a data recovery program and recover part, if not all of the file. In some cases, files may be recoverable months or even years after they've been deleted.
If you are selling your computer, you'll probably want to completely erase the entire hard drive. There are third-party tools which allow you to do this such as Dark's Boot and Nuke. A full format also wipes the hard disk clean by overwriting it with zeros and carrying out a thorough hard disk scan at the same time. Do not confuse a full format with a low level format. A low level format also permanently removes data, but it is not recommended due to the risks and complications involved.