Remove the hdd. Send your friend the pc. He swaps out what he needs, sends it back. You reinstall the hdd. If you have the windows CD, stick that in and in bios make sure the boot order is the optical drive first. Boot from the CD and have windows do a complete fresh install which starts out with a format of the hdd. When it warns you that this'll destroy all prior data, say yes, you want to.
This'll put windows onto a working pc, which will make the pc easier to sell for a slightly higher price, while dumping any and all of your personal info.