Completely Erase the Hard Disk through Disk Wiping:

So it is necessary for us to use a 100 percent secure way to erase the data from the hard disk drive. This way of securely erasing the data is called Disk Wiping. Disk wiping is a secure method of ensuring that data, including company and individually licensed software on your computer and storage devices is irrecoverably deleted before recycling or donating the equipment. Because previously stored data can be brought back with the right software and applications, the disk wiping process will actually overwrite your entire hard drive with data, several times. Once you format you’ll find it all but impossible to retrieve the data which was on the drive before the overwrite. The more times the disk is overwritten and formatted the more secure the disk wipe is.

There are a variety of disk wiping products available that you can purchase, or freely downloaded online to perform more secure disk wipes. One of my favorite disk wiping software is:

WipeDrive/ WipeDrive Professional

You have to use this tool by burning the iso image file onto a CD or by using a floppy disk. After burning this tool you have to boot your PC and follow the screen instructions to completely wipe out the data from your hard disk drive.