Using any live CD
1. Open the terminal
2. Find the hard drive by typing this command: fdisk -l
3. Assuming that the drive is labeled “SDA”, type the following command: dd if=/dev/zero of=/dev/sda bs=1M
This will wipe your drive by writing zeros from the beginning to end. This will take a long time and no progress will be shown, when it is done it will return you to the command prompt.
Alternatively, you can download Darik’s Boot n Nuke, and use that utility to wipe your disk.
with both of these methods it is very important to pick the right disk . if at all possible make sure it is the only disk in your computer.