If you are thinking to sell your computer or only its hard disk then maybe it would be nice to know how to wipe your computer data.
After you followed this procedure there will be no way to recover your data.
The magic command is here, you can use LiveCD (for example Knoppix):
#shred -vz /dev/sda
# shred -u -n 30 /dev/hda
This will overwrite the data on the drive with garbage using 30 passes. The drive will need to be re-formatted after this as even the filesystem structure will be destroyed.
If you want to read more about this command try man shred or info shred