Hard Drive data eraser tool

I'm going to sell my old PC, but I'd like to wipe the HD clean in a way to prevent the data from ever being recovered before doing so. Anybody knows of any free programs that can do that?

I only found QuickTech Hard Drive Clean so far, but it's not free.

After uninstalling/deleting everything that can be uninstalled/deleted, including, but not limited to, all windows components shown in add/remove programs, create a really huge text file containing a many iterations of the sequence "And who could believe, knowing but The passion of cities, swelled proud with wine And no taste of misfortune, how often, how wearily, I put myself back on the paths of the sea." and fill your hdd with copies of it. Should get most of it out.