To permanently wipe data, you can physically destroy hard drive, or use software to achieve. If you are good at command line, you can use Windows build-in program "Cipher Tool" (Cipher.exe) to wipe hard drive. But for those who are not, you could use Partition Assistant Standard Edition (one free and perfect 3rd-party partition software). It is portable and easy to use, while ensuring deleted data unrecoverable. By this software, every sector of the hard drive will be overwritten with binary 1's and 0's. You can even overwrite your hard drive multiple times for added protection.
1. Open Partition Assistant Standard Edition, right click on disk you want to wipe, and then click "Wipe Hard Drive…"
With the feature, you can wipe hard drive partition that already existed.
With the feature, Partition Assistant Standard Edition can help you wipe the unallocated space when you had deleted partition as well.
However, you should notice that, depending on the amount of data and property of the hard drive, it may take a long time to wipe hard drive or require restarting. And once your data have been cleaned up, it will never recovered by any data recovery program.
Partition Assistant Standard Edition is freeware, which supports Windows 2000/XP/Vista/7 (32bit). For pro users, you could select a paid edition for your business. Buy Partition Assistant Pro Edition!