It is always advisable to securely erase or delete your private and confidential data, before selling out your Hard disks or disposing them.
HDShredder comes as a self-booting application which can even run without a working operating system. You can easily create a bootable CD/DVD on Windows to use it.
Download HDShredder 3.6 Free Edition
Also See: Disk Wipe – Portable tool to remove data permanently from HDD & USB devices