Are you planning on giving your old computer away? Well before you dump it off on family or charity, wipe that drive clean.
Darik's Boot and Nuke tool is a very simple way of cleaning your hardrive of your personal files. Just boot up the machine from the Darik's boot disk and watch it wipe all the drives.
I used this tool to clean about 20 computers that I donated to goodwill. Works like a champ.