from a linux command prompt you could use
Code:
dd if=/dev/zero of=/dev/hdX bs=1k
(where X is the number of the hard drive, hda for prim. master)
This will effectively overwrite the complete hard drive (including the partition table) with zeroes.
This will prevent 'normal' users from finding anything on your drive.
If you want to be really sure you could do a
Code:
dd if=/dev/random of=/dev/hdX bs=1k
to overwrite the complete harddisk with random data.

Afterwards repartition and reinstall or physicly destruct the harddrive.

For the government standard on data destruction you would have to repeat this process a bunch of times.