So do you want to wipe the whole hard drive? Or do you have two OSes and want to keep one and wipe the other?
In the first case, boot with any linux live CD and run
dd if=/dev/zero of=/dev/sda (assuming /dev/sda represents your harddrive, you could check this out with fdisk -lu, e.g.)
This will overwrite each block with a 0, once (which is enough, no matter o dusicussion overwriting n times to be safe).
In the second case it's more or less the same, just change the of=/dev/sda to the partition linux is stored, e.g. /dev/sda2.
But you also need to take care of the bootloader...