Use shred to wipe your old computers hard drive before putting it in a dumpster

Boot from a Knoppix CD and then:

# get root
su -

# Find out what hard disks are installed by using fdisk minus L
fdisk -l

Disk /dev/sda: 200.0 GB, 200049647616 bytes
255 heads, 63 sectors/track, 24321 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x0002730b


   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1       24321   195358401   83  Linux

# in the above output the hardisk is /dev/sda. 
# This can change depending on what type of harddisk is installed
# On older PC's it's usually /dev/hdY (where why can be a, b, c etc)

# to wipe a partition use /dev/sdaX (where X is the partition number)
# or the whole disk /dev/sda
shred -n 1 -z /dev/sda

You can also just zero fill a partition using dd thusly:

dd if=/dev/zero of=/dev/sdaX
# where X is the partition you wan't to blat