How do I erase the data in a Windows 7 running laptop completely and make it irretrievable?

I am going to exchange my old laptop. And I would like to know steps of precautions to be taken before handing it over.
Method 1
  1. Backup your data.
  2. Download Knoppix Linux.
  3. Burn it to a CD/DVD/USB.
  4. Boot from it.
  5. Open a terminal.
  6. Type
    sudo dd if=/dev/zero of=/dev/sda
    and hit Enter.
  7. Wait.
  8. You're done!
Method 2 (if the first doesn't work):
Warning: This utility charges money for guaranteed erasing or for SSD erasing.
  1. Backup your data.
  2. Download dban (Darik's Boot And Nuke), a popular computer wiping utility.
  3. Burn it to a USB drive and boot from it.
  4. Type
    in the prompt and hit Enter.
  5. Wait.