How do I completely wipe hard drives before disposal

I am trying to wipe clean the hard-drives on a desktop PC before disposing of the machine. I have tried two freeware programs, but neither of them seems to work because the machine apparently lacks the capacity to open them. The programs download onto a memory stick as ISO files, which the PC does not recognise. Peter Browning, by email You are almost there. An ISO file is a container for a set of files. These are used to create a bootable disc (or memory stick) that starts the computer, instead of Windows, and in your case, runs the disc wiper program. There is one more step and you need a handy freeware program called ImgBurn (download link at: http://goo.gl/TZGVU). It is very easy to use, simply pop in a blank disc or memory stick, select the ISO file from your downloads folder and click the Create Image File from Disc icon. Once it has finished restart the PC and it should boot from the disc or drive. If not you need to run your PC’s BIOS or Start Up program (see manual for instructions) and change the boot order, so that first boot device is your CD/DVD drive or USB drive.