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):
dd if=/dev/zero of=/dev/sda bs=1M
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!!!