Boot just about any Linux Live CD/DVD. Assuming that the disc is the system drive (/dev/sda), you would do this once you have booted the disc and logged in (as root):
[code]
dd if=/dev/zero of=/dev/sda bs=1M
[/code]
This will erase your drive 1MB at a time until it is totally wiped. You can use greater or lesser values for 'bs', but I have found from experience that 1M (1 megabyte == 1024x1024 bytes) is the "sweet spot" for performance on most current gear.

WARNING! This will totally erase the drive, including the partition table and boot record! Use with caution!!!

dd if=/dev/zero of=/dev/sda bs=1M

Simply filling a drive with null characters is insufficient to prevent data from being recovered. The process of sanitizing a disk properly invovles slightly more work.

Standards such as HIPAA set out specific requirements for sanitizing hard drives. A typical requirement is to set all bits to 1s, then 0s, followed by overwriting with a random value.

Expose the hard drive to a very strong magnetic field (put it close to car starter motor,) that will really erase anything on the disc (and probably need a manufacturers deep re format!