Ossie, if you go here
http://www.dban.org/ you can download dban to wipe the hard drives of the computers.
Burn it to disc and set your PCs to boot from CD/DVD then follow the prompts - or read this article's dban section
http://www.pcadvisor.co.uk/how-to/pc-co ... ard-drive/You then have the choice to leave them in or take them out - at least you will know that they are clean.
Apart from making sure there are no discs in other drives there is not much else to do.
I usually stick a label on saying which processor and how much memory they have.... to help the "borrowers" amongst us.