make ready to donate

The best option is simply to remove the hard drive before donating -- though this makes the machine less usable though.

 

If you want to give them the machine and the hard drive, a program called DBAN does an effective job of wiping out data on the drive:  http://dban.sourceforge.net/  .  While this will let you donate the drive as well, the machine will be unbootable at this point unless you or them reinstall windows (do you have the CD?).