First off, break any RAID arrays your drives are in. I recommend removing all drives other than the one(s) you intend to secure erase.
Then, download gParted, from here: http://sourceforge.net/projects/gpar...1.iso/download
Burn it to a fresh CD, or if you are technically inclined, you may write it to a USB stick.
Writing to a USB stick leaves the stick in a state where windows cannot use it, and requires decent knowledge of partition tools to restore to usability by windows. I will not help with this, and this is why I recommend making a CD.
Either way, once this is done, boot the CD, and follow the video guide from there:
You'll have to excuse my sexy voice, I have a reconstructed facial injury that makes me sound vaguely like Kermit the Frog.
Command used is as follows:
hdparm --user-master m --security-erase NULL /dev/sda