There is a linux based free (open source and free of cost) live CD called Slax. It runs a complete operating system from your CDROM drive, allowing you to do things like maintenance on the hard drive that you usually run you operating system from.
These are the instructions for using SLAX.
You will need to read them, at least the steps on burning the CD, starting the CD, starting a graphical user environment, and how to use modules. Modules are important, that basically tells you how to add programs to the CD that are not there already.
THen you need to download slax standard edition (http://www.slax.org/download.php) and the module called wipe (http://www.slax.org/modules.php?category=security).
Then, use the instructions above to add the module to your iso and burn the iso to CD.
Once you boot into the slax environment, you can either start the graphical interface or not... it does not really matter. However, you need to work in a command line environment, which you get to in the graphical environment by clicking on the little icon that looks like a command prompt.
You need to know what hard drive you want to mount. If yours is a standard PC with a single hard drive, that will be /dev/hda. The second hard drive will be /dev/hdb and so on. If you have a non-standard system, with RAID arrays or something, that is a bit more detailed. Drop a line and I will help you out. Anyway, type this in your command window:
umount -a (hit return)
wipe /dev/hda (hit return)
This will erase your the first harddrive in your system and then overwrite it 34 times with patterns designed to make anything that you erased unreadable by anybody ever.