Not possible.

Do one clean installation, windows 98, word, excell, ... Then ghost it to all pc's.

If they are branded pc's they might have a cd with an utility to put them back to how they were factory delivered.
(compaq is a cd, old dell is zztop utility, some newer is F12 when starting up pc, then choose option, ....)

Best option is to get someone to help you that knows more then the "on" button though, otherwise you'll be working ages on those 20 pc's.