1: Boot from a Knoppix LiveCD and use the 'shred' utility
2: Use DBAN - Darik's Boot and Nuke
Using Knoppix ought to give you more control over the process but I found it to be less than ideal. This is how you would use it:
1: Boot the machine from a burned copy of the Live CD
2: Open a console in the desktop that appears
3: Run shred with suitable parameters on each partition on your machine, e.g.:
# shred -n 10 -z -v /dev/hdbThis would run 10 rounds of writing random date to the specified disk (-n 10), followed by one round of writing zeroes (-z).
The problem I had was that I didn't remember the disk ids on that machine. Normally you could just run 'fdisk -l' and see what was there... BUT the Knoppix version of fdisk doesn't give you that - dang it - so I had to reboot the machine under its own linux and see what was there. That was annoying - must be another way to do that but I don't know it.
DBAN does what it says on the tin. It boots a machine and then nukes the contents of every disk it can find. It is designed for disk erasing and gives you various degrees of erasure, including the Dept. of Defense standards.
1: Boot your machine with a CD version of the DBAN iso (only 2MB!)
2: Look at the options or just type 'autonuke' at the boot prompt (this gives you the default 'DoD short' level of erasing.
3: Go away and do something else - it takes ages (as in an hour or more)
One thing to note - the current version (as of June 2009) is 1.0.7 - the DBAN site mentions that for Core 2 Duo processors you need version 2 which is a pre-release at this stage.