Hi,
I would find a freeware drive eraser from the internet that wipes to military standards, then run it on each machine.
Do a google search for hard drive eraser and see what comes up.
Depending upon the computer literacy of the recipients of the machines, an alternative of course would be to re image the machines from a clean setup. This wouldn't stop a technical expert running data recovery software over it, but would beat about 99% of the population.
John