Go to the link posted below and download (free) DBAN disk wiper. It will be an image (ISO) and then burn it to a CD or DVD disk. IMPORTANT: Make sure you have the BIOS set to CD=First Boot Device.
Now insert the CD or DVD in your driver and reboot. Choose "Automatic". Let it run, it will take a while so go and watch a movie or something. It will wipe the drive including partition. It would take the equivalent of the FBI to recover any data off the hard drive once it is finished, this is a pro low-level data remover.
click the red "DOWNLOAD" button.
Incidentally, before you install Windows7 I strongly suggest you run the (free) Microsoft system analyzer to make sure W7 will run on your legacy system.
If there are further questions, I hang out at www.suggest-a-fix.com
which is a free computer help forum. Post in the Hardware section and I will be along. This forum is not set up for complicated problems that require repeated, in-depth communication.