You can't. What I would suggest doing, is getting a program like DBAN and run a pretty aggressive disk wipe routine on it. Then either donate the computer with the restore media, or sans hard drive. If you're feeling generous you can reinstall Windows onto the drive after the secure wipe, but there is always a small chance data could be retrieved even then. It may be a very small chance, bordering on impossible, but a chance none the less. So you need to decide if that is an acceptable risk to you.
I am giving away some computers and need to delete all my information and files on the computer. How do I delete "everything" except the operating system and anything needed to run that operating system.