The default shred command in knoppix is better than dd if=/dev/zero of=/dev/hda in my opinion. To start, shred will show you progress and what it's overwriting the data with (random, zeros, etc). Second, you can specify the number of times it rewrites the data. All formatting is also erased.
Simply enter the following command:
sudo shred -n 2 -z -v /dev/hda
This shred command will overwrite the drive twice (-n 2). Random data to start (defalut), zeros to finish (-z). The -v tells you the progress on the drive.